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19.16: Labor and Birth - Biology

19.16: Labor and Birth - Biology


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Labor is the physical efforts of expulsion of the fetus and the placenta from the uterus during birth (parturition). As more smooth muscle cells are recruited, the contractions increase in intensity and force.

There are three stages to labor. During stage one, the cervix thins and dilates. This is necessary for the baby and placenta to be expelled during birth. The cervix will eventually dilate to about 10 cm. During stage two, the baby is expelled from the uterus. The uterus contracts and the mother pushes as she compresses her abdominal muscles to aid the delivery. The last stage is the passage of the placenta after the baby has been born and the organ has completely disengaged from the uterine wall. If labor should stop before stage two is reached, synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor.

An alternative to labor and delivery is the surgical delivery of the baby through a procedure called a Caesarian section. This is major abdominal surgery and can lead to post-surgical complications for the mother, but in some cases it may be the only way to safely deliver the baby.

The mother’s mammary glands go through changes during the third trimester to prepare for lactation and breastfeeding. When the baby begins suckling at the breast, signals are sent to the hypothalamus causing the release of prolactin from the anterior pituitary. Prolactin causes the mammary glands to produce milk. Oxytocin is also released, promoting the release of the milk. The milk contains nutrients for the baby’s development and growth as well as immunoglobulins to protect the child from bacterial and viral infections.


Teenage Pregnancy

Teenage pregnancy is when a woman under 20 gets pregnant. It usually refers to teens between the ages of 15-19. But it can include girls as young as 10. It's also called teen pregnancy or adolescent pregnancy.

In the U.S., teen birth rates and number of births to teen mothers have dropped steadily since 1990. In 2018, just under 180,000 infants were born to teens 15 to 19 years old. Teen birth rates have fallen by 70% in the past 3 decades. That trend is driven both by the fact that fewer teenagers are having sex and that more of them use birth control when they do.

Even so, a much higher share of American teens get pregnant than girls in other developed countries. And the pace of the decline in teen pregnancy in the U.S. differs by race. Non-Hispanic Black girls and Native American girls have seen much slower drops in teen pregnancy compared to Asian American girls.

Here’s what to know about common early signs of pregnancy, how to have a healthy pregnancy at a young age, and information that will help you understand teenage pregnancy.


The early stages of pregnancy

Following conception, a new embryo must signal its presence to the mother, allowing her body to identify the start of pregnancy. When an egg is fertilised, it travels though the female reproductive tract and on day six implants into the womb releasing a hormone called human chorionic gonadotrophin in the process. This hormone enters the maternal circulation and allows the mother to recognisethe embryo and begin to change her body to support a pregnancy.

Human chorionic gonadotrophin can be detected in the urine as early as 7-9 days after fertilisation and is used as an indicator of pregnancy in most over-the-counter pregnancy tests. It is partly responsible for the frequent urination often experienced by pregnant women during the first trimester. This is because rising levels of human chorionic gonadotrophin causes more blood to flow to the pelvic area and kidneys, which causes the kidneys to eliminate waste quicker than before pregnancy. Human chorionic gonadotrophin passes through the mother’s blood to the ovaries to regulate the levels of the pro-pregnancy hormones, oestrogen and progesterone.


During labor, certain hormones are released that cause the contraction of the uterus in order to push the baby out. When those hormones are sensed by the glands of the endocrine system, they secrete more of that hormone which keeps labor going until the baby is born.

The endocrine system assists in maintaining and regulating different functions of the body by producing and discharging hormones. It comprises glands situated throughout the body, which produce chemicals known as hormones directly into the blood. The levels of hormones in the blood are monitored by a highly unique homeostatic mechanism known as feedback.

The two kinds of feedback are common, that is, negative and positive feedback, of this positive feedback, is the rare mechanism. It augments the changes done rather than opposing them. For example, the discharging of oxytocin from the posterior pituitary gland at the time of labor is an illustration of a positive feedback mechanism.

The stimulation of the muscle contractions, which pushes the baby via the birth canal is done by oxytocin. This discharging of oxytocin leads to an augmented or stronger contraction at the time of labor. This contraction is enhanced and intensify until the baby comes out of the birth canal. However, when the stimulus to the pressure receptor terminates, the generation of oxytocin ceases, which eventually results in the stopping of labor contractions.


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BRANCHBURG, N.J., April 19, 2016 &ndash The Christie Administration today joined the New Jersey Business & Industry Association (NJBIA) at its first annual Workforce Development Summit where business and workforce leaders met to discuss workforce development investments targeted at meeting industry needs. Deputy Commissioner Aaron R. Fichtner, Ph.D of the New Jersey Department of Labor and Workforce Development (LWD) moderated a panel discussion on advancing workforce skills and highlighted the department&rsquos new Talent Network initiative focused on meeting business demands for skilled workers.

&ldquoMy department is committed to strengthening our workforce by partnering with businesses to determine their workforce needs and skill gaps,&rdquo said Department of Labor Commissioner Harold J. Wirths. &ldquoOur new Talent Network initiative will further our mission of determining workforce investments that produce workers prepared to further their careers and strengthen our businesses.&rdquo

Throughout 2016, the state&rsquos seven Talent Networks will facilitate the development of new high-quality employer-driven partnerships know as Targeted Industry Partnerships (TIPs) throughout the state. The Talent Networks will work closely with employers to identify industry-valued credentials for workers and define employer training and skill needs. The Talent Networks will then assemble workforce and education stakeholders to assess capacity and facilitate the development of specific area workforce strategies. These strategies will summarize key industry workforce needs for each TIP and propose efforts to prepare a skilled workforce and the development of new career pathways for students and jobseekers. LWD is committing a minimum of $5 million to support the implementation of the strongest ideas developed through this process.


U.S. Coronavirus Advice Is Failing Pregnant Women

An anthropologist explores how her current study of COVID-19 and childbirth reveals profound and amplified problems with the United States’ maternity system.

A s much as it feels like day-to-day life has been suspended in the time of the COVID-19 pandemic, one thing cannot be put on pause: childbirth. On average, more than 10,000 babies are born every day in the United States.

I n response to the COVID-19 crisis, the U.S. Centers for Disease Control and Prevention (CDC) and the American College of Obstetricians and Gynecologists (ACOG) released guidelines stating that health care facilities, on a case-by-case basis, may consider separating women with suspected or confirmed COVID-19 from their newborns until there is no longer a concern for transmission.

T hese recommendations are substantially more cautious than advice from the World Health Organization (WHO), which does not mention any form of separation but instead emphasizes the importance of early breastfeeding, skin-to-skin contact, and room-sharing with newborns for mothers infected with COVID-19. New breastfeeding guidelines from WHO specifically say not to separate mother and baby when a mother is well enough to breastfeed, even in cases of confirmed or suspected COVID-19.

U nfortunately, the U.S. maternity care response to the COVID-19 crisis is not really so surprising: It is emblematic of the standard—and misguided—“better safe than sorry” policies in the United States for maternity care.

A s a biological anthropologist who has studied the intergenerational effects of maternal stress for the last 12 years, it is clear to me that U.S. policies treat childbirth as a sickness rather than as a natural process shaped by millions of years of cultural and biological evolution.

W hile hospitals and modern medicine save the lives of women and newborns who face serious medical complications, the fact is that most pregnancies are low risk and do not involve such difficulties. The health care environment in the United States, however, is set up so that most pregnant women are funneled into receiving the same medicalized care, regardless of risk.

C-section deliveries are twice as common in the United States as what WHO estimates is likely to be medically necessary. Jonathan Borba/Pexels

T he result is shockingly high rates of medical intervention in the U.S. for women giving birth: Cesarean section, for example, increased by 53 percent from 1996 to 2007. The current rate of 32 percent is more than twice what WHO estimates as the percent likely to be medically necessary. Other types of interventions that many say are overused include restrictions on eating and drinking in labor, the use of continuous electronic fetal monitoring, speeding up labor through drug administration, and episiotomies.

C ritics say the overuse of these interventions not only causes the United States to spend more on maternity care than any other country in the world, but that it also results in physical and emotional costs to women.

D espite the high costs of the U.S. medical system, the country experiences the highest rate of maternal mortality among a set of 10 similarly high-income countries. For every 100,000 live births, 14 American women die during pregnancy, at birth, or within about a month of childbirth—that’s significantly more than in the United Kingdom, where 9 women die for every 100,000 babies.

A ll of these problems have been amplified and highlighted as the coronavirus sweeps through the United States. I am currently co-leading a study with Dartmouth College biological anthropologist Theresa Gildner that evaluates the impacts of the COVID-19 pandemic on pregnant women’s well-being and health care experiences. Our preliminary results throw a spotlight on these issues.

(RE)THINK HUMAN

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T here are several core problems with the current advice in the U.S. on birthing during the pandemic.

F or one, the CDC recommends that the “costs and benefits” of separating mothers from their newborns should be discussed with the pregnant mother—with women being left to make the decision. However, in our study, women reported being unclear about whether or not they are allowed to actually refuse separation from their child after it is born. Even before the pandemic, many women—and in particular women of color—often reported feeling disempowered about their birthing experience. The CDC’s recommendations could make this situation worse.

Many women today don’t experience pregnancy and birth as a natural process. George Jr. Kamau/Pexels

T he potential benefit of separating infants is that it may reduce their risk of contracting COVID-19. Some newborns have tested positive for COVID-19 and even, tragically, died. But the risks and the impacts of COVID-19 for all infants, including those without preexisting medical conditions, remain unclear.

T he cons associated with maternal-newborn separation, however, are very clear. Experts consider immediate skin-to-skin contact essential for helping newborns regulate their body temperature after birth, and it’s associated with lower mortality. Mothers who are able to immediately practice skin-to-skin contact breastfeed their infants longer, which is important since breast milk is known to be good for an infant’s immune system. So, it could be helpful for newborns who might be exposed to infection.

At the height of the pandemic along the East Coast, some hospitals forced women to labor alone.

M other and newborn separation can also hurt maternal emotional well-being, contributing to postpartum depression and even post-traumatic stress disorder. For example, in one 2019 pre-pandemic study, mothers of very premature infants cared for in single-family rooms (as opposed to an open bay unit, a form of separation) had significantly lower depression scores. Both mothers and fathers of the single-family room infants reported less stress during hospitalization.

M aternal contact—including smell, touch, and voice—can impact a child’s physiological responses to stress and, potentially, their later health. One study reported that 2-day-old newborns who slept alone had a 176 percent increase in their stress response relative to newborns who were sleeping in skin-to-skin contact with their mothers.

I t is hard to weigh the medical pros and cons, and as an anthropologist, it is not my job to say what the right approach is. But it is clear that the U.S. recommendations are more cautious than WHO’s, emphasizing possible harms over benefits when it comes to mothers who tested positive for or are suspected of having COVID-19 staying with their newborns.

N otably, the CDC has also recommended that health care facilities consider limiting to one the number of support persons available to women in labor—forcing her to choose between a partner, parent, sibling, friend, or doula. At the height of the pandemic along the East Coast, some hospitals forced women to labor alone. Among women who have participated in our COVID-19 study, many have described being told that they will have to deliver via cesarean without loved ones in the room or that they will need to labor alone if they are COVID-19 positive.

E liminating support teams comes with costs. Evidence shows that having continuous support throughout labor improves both maternal and newborn outcomes, including a better chance of women having a vaginal delivery (as opposed to a caesarian section), fewer interventions with instruments like a vacuum and forceps, and better newborn health.

T rained support people, such as doulas, are particularly helpful because they serve as both emotional support and advocates for women during delivery. Not counting doulas as essential staff could disproportionately affect women of color, who are already more likely to be treated poorly by hospital staff.

M edia stories have reported that fears surrounding COVID-19 have led to an increased interest in alternatives to hospital birth—from home birth to birthing centers, often with the attendance of a midwife.

T hose kinds of births aren’t common in the United States: Only 10 percent of births are attended by midwives, with 1.6 percent of women giving birth at home (usually with midwives). In contrast, in New Zealand, midwives are the lead maternity caregivers for almost 92 percent of pregnancies, and 4 percent of births occur at home.

E vidence suggests that for women with uncomplicated pregnancies, the chance of having an intervention-free birth is higher when women deliver with a midwife in a birthing center or at home than in the hospital. The U.K.’s National Health Service and the Ministry of Health in New Zealand explain that home birth can be a reasonable option for women who are low-risk. In contrast, ACOG guidelines state that hospitals and accredited birth centers are the “safest” setting for birth.

Sleeping skin to skin with mom has known biological benefits for newborns. Nelson Kwok/Flickr

U nfortunately, structural factors have long prohibited access to non-hospital care for many U.S. women. Private insurance companies often do not reimburse for home births, and Medicaid, which pays for just under half of all births, only covers home birth in approximately half of the states. Women who decide to give birth in their home may have to pay for that out of pocket. In our COVID-19 study, many participants stated that despite a preference for home birth, they were unable to follow their wishes due to cost or because they perceived home births to be illegal in their state.

S tand-alone birthing centers, which only cater to laboring mothers and provide more home-like settings, could also be attractive options for individuals seeking an alternative to hospital-based maternity care. A large study of 46,000 Medicaid recipients found that among women with similar risks, those who received care at birth centers were less likely to have babies born preterm, less likely to have a cesarean, and twice as likely to have a vaginal birth after having had a cesarean. What’s more, the cost for care for women going to birth centers was nearly US$2,000 less than average. Stand-alone birthing centers have been rapidly popping up over the last 10 years, but there are still fewer than 400 in the entire United States some states have none.

P art of the reason for this is that certified nurse midwives are not independent practitioners in all states, meaning that midwives need to work with or under a physician if they want to open a birthing center. Some Certificate of Need laws state that hospitals (which would be competitors of such birth centers) need to support centers opening.

T he result is that access to free-standing birthing centers, and midwifery care more generally, is alarmingly limited in the United States.

T he coronavirus pandemic has raised public awareness about the conditions many U.S. women are forced into while giving birth.

Birthing centers can provide an appealing alternative to hospitals for some low-risk pregnant women. Shooting Chris/Flickr

T he general approach to maternal and newborn care in the United States is not based on understanding individual women’s preferences and risk. Instead, it has arisen from the unfounded assumption that more technology, medicine, and intervention in childbirth is “safer.”

T here are some immediate and practical steps that should be taken to improve things during the pandemic: for example, getting pregnant women out of hospitals and into dedicated birth centers, enacting policy changes to ensure that insurance covers home births, and allowing all qualified midwives to act as independent practitioners in every state. In addition, doulas without symptoms or those who test negative for COVID-19 should also be considered as part of a pregnant woman’s care team, not counting as the one allowable companion for a laboring mother. On a promising note, the governor of New York launched a state task force to determine how to authorize additional birthing centers that women could go to instead of hospitals.


The real reasons why childbirth is so painful and dangerous

Giving birth can be a long and painful process. It can also be deadly. The World Health Organization estimates that about 830 women die every day because of complications during pregnancy and childbirth &ndash and that statistic is actually a 44% reduction on the 1990 level.

"The figures are just horrifying," says Jonathan Wells, who studies childhood nutrition at University College London in the UK. "It's extremely rare for mammalian mothers to pay such a high price for offspring production."

So why exactly is childbirth so risky for humans? And is there anything we can do to further reduce those death rates?

Scientists first began thinking about the problem of human childbirth in the middle of the 20th Century. They soon came up with an idea that seemed to explain what was going on. The trouble began, they said, with the earliest members of our evolutionary lineage &ndash the hominins.

From an early date in our prehistory, hominin babies may have had to twist and turn to pass through the birth canal

The oldest hominin fossils so far found date back about seven million years. They belong to animals that shared very few of our features, except perhaps one: some researchers think that, even at this early stage, hominins were walking upright on two legs.

To walk on two legs efficiently, the hominin skeleton had to be pushed and pulled into a new configuration, and that affected the pelvis.

In most primates the birth canal in the pelvis is relatively straight. In hominins, it soon began to look very different. Hips became relatively narrow and the birth canal became distorted &ndash a cylinder that varied in size and shape along its length.

So from an early date in our prehistory, hominin babies may have had to twist and turn to pass through the birth canal. This would have made birth a far more difficult task than it had been previously.

Then things got even worse.

About two million years ago, our hominin ancestors began to change again. They lost their more ape-like features such as a relatively short body, long arms and small brain. Instead they began to gain more human-like ones, like taller bodies, shorter arms and bigger brains.

That last trait in particular was bad news for female hominins.

I was going to find evidence that supported the obstetric dilemma, but very soon everything came crashing down

Big-brained adults start out life as big-brained babies, so evolution came into conflict with itself. On the one hand, female hominins had to maintain a narrow pelvis with a constricted birth canal in order to walk efficiently on two legs. But at the same time the foetuses they carried were evolving to have larger heads, which were a tighter and tighter fit through those narrow pelvises.

Childbirth became a distressingly painful and potentially lethal business, and it remains so to this day.

In 1960, an anthropologist called Sherwood Washburn gave this idea a name: the obstetrical dilemma. It is now often called the "obstetric dilemma". Scientists thought it explained the problem of human childbirth perfectly. Many still think it does.

But some, including Wells, are no longer happy with this standard explanation. In the last five years, Wells and several other researchers have begun to push against the classic story of the obstetric dilemma.

They think Washburn's idea is too simplistic, and that all sorts of other factors also contribute to the problem of childbirth.

Holly Dunsworth of the University of Rhode Island, Kingston, was drawn to the obstetric dilemma while she was still a grad student. "I thought it was so exciting, I was going to find evidence that supported the obstetric dilemma," she says. "But very soon everything came crashing down."

We have bigger babies and longer pregnancies than you would expect

The problem was with the predictions Washburn made. "When Washburn wrote his article, he was actually saying that the obstetric dilemma was solved by giving birth to babies at a relatively early stage in their development," says Wells.

Go back to that moment two million years ago when human brains began to grow larger. Washburn suggested that humans found a solution of sorts: shortening the length of the human pregnancy. Human babies were forced out into the world earlier than they really should be, so that they were still relatively small, with diminutive, underdeveloped brains.

Washburn's explanation seems logical. Anyone who has held a newborn can appreciate how underdeveloped and vulnerable they are. The standard view is that other primates hold onto their pregnancies for longer and give birth to babies that are more developmentally advanced.

But, says Dunsworth, it is simply not true.

"We have bigger babies and longer pregnancies than you would expect," she says.

Women give birth to babies with larger brains than we would expect

In an absolute sense human pregnancies are long. They typically last 38-40 weeks, whereas a chimpanzee pregnancy is 32 weeks long, and gorillas and orang-utans give birth after about 37 weeks.

As Dunsworth and her colleagues explained in a 2012 paper, this remains true even if we adjust the pregnancy durations to take into account differences in body mass. Human pregnancies last 37 days longer than they should do for an ape our size.

The same thing applies for brain size. Women give birth to babies with larger brains than we would expect of a primate with the average woman's body mass. This means that a key prediction of Washburn's obstetric dilemma is incorrect.

There are other problems with Washburn's idea too.

A central assumption of the obstetric dilemma is that the size and shape of the human pelvis &ndash and the female pelvis in particular &ndash is highly constrained by our habit of walking upright on two legs. After all, if evolution could have "solved" the problem of human childbirth by simply making women's hips a little wider and the birth canal a little larger, it surely would have done so by now.

The birth canal is extraordinarily variable in size and shape

In 2015, Anna Warrener at Harvard University in Cambridge, Massachusetts, and her colleagues questioned this assumption.

The researchers collected metabolic data from male and female volunteers who were walking and running in the lab. Volunteers with wider hips were no more inefficient at walking and running than their narrow-hipped peers. From purely energetic considerations, at least, there does not seem to be anything stopping humans evolving wider hips that would make childbirth easier.

"The basic premise of the obstetric dilemma &ndash that having a small or narrow pelvis is best for biomechanical efficiency &ndash is likely not correct," says Helen Kurki of the University of Victoria in British Columbia, Canada.

Kurki was not involved with Warrener's study, but her own research has identified yet more problems for the traditional obstetric dilemma hypothesis.

If the female pelvis really is tightly governed by two opposing forces &ndash the need to be narrow for walking and the need to be wide for giving birth &ndash the shape of the birth canal should vary little between women. It should be "stabilised" by natural selection.

Pregnant women sometimes joke that their developing foetus feels like an energy-sapping parasite

But after analysing hundreds of human skeletons, Kurki reported in 2015 that the birth canal is extraordinarily variable in size and shape. It varies even more than the size and shape of human arms, a trait that is known to vary between individuals.

"I think my findings do support shifting attitudes to the obstetric dilemma," says Kurki.

Washburn's tidy narrative does not seem quite as satisfying as it once did. There has to be something else going on.

Dunsworth thinks she has identified one important missing piece in the puzzle: energy.

"We max out toward the end of pregnancy," says Dunsworth, herself a mother. "Those last weeks and months of pregnancy are tiring. They are pushing right against the possible sustainable metabolic rates in humans. It has to end at some point."

Evolution could, in principle, make the pelvis larger &ndash but it has not had to

Pregnant women sometimes joke that their developing foetus feels like an energy-sapping parasite. In a sense it really is, and its energy demands grow with every passing day.

In particular, human brains have an almost insatiable appetite for energy. Growing a second, tiny brain inside the womb can push a pregnant woman close to the edge, metabolically speaking.

Dunsworth calls this idea the energetics of gestation and growth (EGG) hypothesis. It suggests the timing of childbirth is governed by the difficulties of continuing to nourish a developing foetus beyond 39 weeks &ndash not by the difficulties of squeezing the baby out through the birth canal.

Dunsworth thinks people obsess too much about the tight fit between a baby's head and its mother's birth canal. It might seem too much of a coincidence that the two are so closely size-matched, but she says the pelvis has simply evolved to be the size it needs to be. Evolution could, in principle, make the pelvis larger &ndash but it has not had to.

For most of human evolution, childbirth might have been quite a lot easier

By and large, Kurki shares this view. "The obstetric canal is big enough, the majority of the time, for the foetus to pass through," she says.

This is true. But even so, take another look at the maternal mortality figures: 830 deaths every day. Even among women who do not lose their lives during childbirth, some studies say the process leads to life-changing but non-lethal injuries in as many as 40% of cases. The price women pay for childbirth seems extraordinarily high.

Wells agrees. "It's impossible to imagine the problem has been this bad over the long term."

Perhaps it has not. In 2012, Wells and his colleagues took a look at the prehistory of childbirth, and came to a surprising conclusion. For most of human evolution, childbirth might have been quite a lot easier.

The prehistory of childbirth is a difficult subject to study. The hominin pelvis is rarely preserved in the fossil record, and newborn skulls are even thinner on the ground. But from the meagre evidence available it seems that some earlier species of human, including Homo erectus and even some Neanderthals, had a relatively easy time of it when it came to giving birth.

A shift to farming may have led to developmental changes that made childbirth far more difficult

In fact, Wells and his colleagues suspect childbirth might even have been a relatively minor problem in our species &ndash at least to begin with. There are very few newborn baby skeletons among the human remains from early hunter-gatherer groups, which might hint that death rates among newborns were relatively low.

If there was a rise in newborn death rates at the dawn of farming, there were almost certainly several factors involved.

For instance, early farmers began living in relatively dense settlements, so transmissible disease probably became a far greater problem. Newborns are often particularly vulnerable when an infection is going around a community.

But Wells and his colleagues suspect a shift to farming also led to developmental changes that made childbirth far more difficult. A rise in infant mortality at the dawn of farming might be due in part to a raised risk of death during childbirth.

Human childbirth suddenly became more difficult about 10,000 years ago

There is one striking feature archaeologists have noticed when comparing the skeletons of early farmers with their hunter-gatherer ancestors. The farmers were noticeably shorter in stature, probably because their carbohydrate-rich diet was not particularly nutritious compared to the protein-rich hunter-gatherer diet.

This is a telling observation for those who study childbirth, says Wells, because there is evidence of a link between a woman's height and the size and shape of her pelvis. In general, the shorter a woman, the narrower her hips. In other words, the shift to farming almost certainly made childbirth a little bit more challenging.

On top of that, the carbohydrate-rich diets that became more common with farming can cause a developing foetus to grow larger and fatter. That makes the baby harder to deliver.

Combine these two factors and human childbirth &ndash which might have been relatively easy for millions of years &ndash suddenly became more difficult about 10,000 years ago.

Something rather like this "farming revolution effect" replays whenever human diets become poorly nutritious &ndash particularly if those diets also contain a lot of carbohydrates and sugars, which encourage foetal growth.

"We can make a simple prediction that the nutritional status of mothers should be associated with a local prevalence of maternal mortality and difficulties with giving birth," says Wells. The statistics clearly follow such a pattern, suggesting that improving nutrition might be a fairly easy way to reduce maternal mortality.

Pregnant women have adapted to nourish their foetus for as long as they can

Both Dunsworth and Kurki think that Wells has identified something significant in his work &ndash something that perhaps would only be evident to a researcher with the right background in nutrition and development.

"I'm so lucky that Jonathan is describing these complex issues from his perspective of human health," says Dunsworth. "At the same time I'm approaching the problem from my perspective of human evolution."

So we now have a new explanation for the difficulties of human childbirth. Pregnant women have adapted to nourish their foetus for as long as they can before it grows too large to feed internally. The female pelvis has adapted to be just the right size to allow this maximally-nourished foetus to travel through safely. And dietary changes in the last few thousand years have upset this fine balance, making childbirth risky &ndash particularly for mothers who have a poor diet.

However, Dunsworth says that is probably not the end of the story.

Washburn's ideas made good intuitive sense for decades, until Dunsworth, Wells, Kurki and others began to pick them apart. "What if the EGG perspective is too good to be true?" asks Dunsworth. "We have to keep searching and keep collecting evidence."

This is exactly what other researchers are doing.

For instance, in 2015 Barbara Fischer of the Konrad Lorenz Institute for Evolution and Cognition Research in Klosterneuburg, Austria and Philipp Mitteroecker of the University of Vienna, Austria took another look at the female pelvis.

A woman's pelvis takes on a shape more conducive to childbirth in her late teens &ndash when she reaches peak fertility

It seemed to them that Dunsworth's EGG hypothesis &ndash compelling though it is &ndash could actually be seen as complementary to Washburn's ideas, rather than disproving them entirely. Dunsworth agrees: she thinks many factors are involved in the evolution of modern childbirth.

Fischer and Mitteroecker investigated whether there is any correlation between female head size and pelvis size. Head size is heritable, at least to some extent, so women would benefit during childbirth if those with larger heads also naturally had a wider pelvis.

The researchers' analysis of 99 skeletons suggested such a link does indeed exist. They concluded that a woman's head size and her pelvic dimensions must somehow be linked at the genetic level.

"This does not mean that the [problem of childbirth] has been resolved," says Fischer. But the problem would be even worse if there was no link between head size and pelvis width.

And there is another complication: women's bodies change as they get older.

A May 2016 study led by Marcia Ponce de León and Christoph Zollikofer at the University of Zurich, Switzerland examined pelvic data from 275 people &ndash male and female &ndash of all ages. The researchers concluded that the pelvis changes dimensions during the course of a woman's lifetime.

Many babies are now born by Caesarean section

Their data suggested that a woman's pelvis takes on a shape more conducive to childbirth in her late teens &ndash when she reaches peak fertility. It then stays that way until around her 40th birthday, when it then gradually changes shape to become less suitable for childbirth, ready for the menopause.

The scientists suggest these changes make childbirth a little easier than it otherwise would be. They call this idea the "developmental obstetric dilemma" (DOD).

"The DOD hypothesis provides a developmental explanation for the variation in pelvic obstetric dimensions," says Ponce de León.

If all these evolutionary pressures are acting on childbirth, is the process still changing and evolving even now?

In December 2016, Fischer and Mitteroecker made headlines with a theoretical paper that addressed this question.

Earlier studies had suggested that larger babies have a better chance of survival and that size at birth is at least somewhat heritable. Together, these factors might lead the average human foetus to push up against the size limit imposed by the female pelvis, even though it can be fatal to push too far.

We all either did or didn't arrive in the world through a pelvis

But many babies are now born by Caesarean section, an operation in which the baby is taken out of the mother's abdomen without ever entering the birth canal. Fischer and Mitteroecker suggested that, in societies where C-sections have become more common, foetuses can now grow "too large" and still have a reasonable chance of survival.

In theory, as a consequence the number of women giving birth to babies that are too big to fit through their pelvis might have risen by 10 or 20% in just a few decades, at least in some parts of the world. Or, to put it in cruder terms, people in these societies might be evolving to have larger babies.

For now this is only an idea and there is no hard evidence that it is really happening. But it is an intriguing thought.

"We all either did or didn't arrive in the world through a pelvis," says Wells. "If we did, that pelvis mattered. And if we didn't, that in itself is interesting."

Ever since live birth evolved, babies have been constrained to some degree by the size of the birth canal. But maybe, for some babies at least, that is no longer true.

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The evolution of modern human childbirth

Human birth follows a pattern which is unique among mammals. Distinctions include the orientation of the fetus as it passes through the birth canal, the way the fetus emerges from the birth canal, difficulty during labor, and behavior by the mother and/or other individuals around the time of birth. Birth has important implications for the morphology of the pelvis, for sex differences in the pelvis, for such aspects of human biology as size (and maturity) at birth, and for behavior (including cooperative behavior). This paper reviews the fossil and comparative evidence for when and how the modern pattern of birth evolved.

The modern human pattern of birth evolved in a mosaic manner with some unique features appearing early in human evolution and others quite late. A human-like entry of the fetal head into the birth canal was already present among australopithecines as a result of their wide pelvic apertures. Other aspects of modern human birth such as the rotation of the head and body within the birth canal and the emergence of the fetal head in an occiput anterior position probably evolved later, when encephalization had placed increasing selection on both the form of the pelvis and the timing of birth. Cooperative behavior during and after birth accompanied the origin of the fully modern human mechanism of birth.

The unique phenomenon of modern human birth did not evolve in response to a single “obstetrical dilemma” but as part of a complex interplay between changes in a number of aspects of human biology. © 1992 Wiley-Liss, Inc.


Recommended Classification of Deliveries From 37 Weeks of Gestation

Early term: 37 0/7 weeks through 38 6/7 weeks

Full term: 39 0/7 weeks through 40 6/7 weeks

Late term: 41 0/7 weeks through 41 6/7 weeks

Postterm: 42 0/7 weeks and beyond

Uniform definitions of term are predicated on a uniform method of determining gestational age. The work group provided a method for determination of gestational age 5 that, like other similar methods 6, focused on a hierarchy of clinical and ultrasonographic criteria. Individual methods may differ in the details of when and how ultrasonographic biometry should be used to change estimated date of delivery based on last menstrual period however, it is not the purpose of this document to establish the priority of one method over another. The College and SMFM are working with other expert groups to establish evidence-based consensus on criteria for determining gestational age.


Fetal-to-neonatal transition: What is normal and what is not? Part 1

The transition from fetus to neonate is a critical time of physiological adaptation. While the majority of term infants complete this process in a smooth and organized fashion, some infants experience a delay in transition or exhibit symptoms of underlying disease.

Careful assessment and astute care is needed during the period of transition to ensure that the neonate who is experiencing problems with transition is recognized and appropriate interventions initiated.

Part 1 of this paper examines the physiology of transition, focusing on the changes occurring in the cardiovascular and respiratory systems. Part 2 will address the signs and symptoms which indicate the need for further evaluation of the infant.

The transition from fetal to neonatal life requires complex physiological changes that must occur in a relatively short period of time.

The fetus must move from reliance on the maternal heart, lungs, metabolic and thermal systems to being able to self-sufficiently deliver oxygenated blood to the tissues and regulate various body processes.

While the majority of critical transitions occur in the first few moments after birth, circulatory and pulmonary changes continue for up to 6 weeks after birth. Transition is a time of significant risk to the newborn and necessitates astute observations on the part of the healthcare team.

While most term infants achieve physiological homeostasis without difficulty, careful assessment during this period of adaptation is required to ensure that the infant makes the transition smoothly and without compromise.

Prompt recognition of those infants who present with signs of serious illness enables caregivers to initiate treatment aimed at minimizing the effects of such illness.

This article will review the physiological adaptations occurring during the transition from fetal to neonatal life and will examine common red flags which may alert care providers to an infant experiencing delayed transition or an underlying disease process, congenital abnormality or birth injury.

PHYSIOLOGY OF TRANSITION Cardiovascular changes

Fetal circulation is characterized by the presence of three shunts, the ductus venosus, ductus arteriosus and foramen ovale, as well as high pulmonary vascular resistance (PVR) resulting from the relative hypoxic pulmonary environment (pO2 17-19 mmHg) and low systemic vascular resistance (SVR) [5].

To fully appreciate the hemodynamic changes which occur after birth, a review of fetal circulation is necessary (Fig. 1).


click to enlarge

FIGURE 1: Neonatal Cardiovascular System (Reprinted by permission from Ross Labs http://www.rosslearningcenter.com/)

In utero, oxygenated blood is delivered from the placenta to the fetus through the umbilical vein and into the liver.

Some of this blood perfuses the liver, while the rest of the blood bypasses the hepatic system through the first fetal shunt, the ductus venosus, which forms a connection between the umbilical vein and the inferior vena cava (IVC).

The percentage of blood directed towards the liver increases with increasing gestational age with about 80 % entering the liver by 32 weeks' gestation [3,14,15].

In the IVC oxygenated blood from the ductus venosus mixes with unoxygenated blood from the lower body although the oxygenated blood, which has a higher level of kinetic energy, tends to remain in a relatively separate stream [5].

When the stream of oxygenated blood enters the right atrium, about 50-60 % is directed through the foramen ovale to the left atrium by the Eustatian valve (a flap of tissue at the IVC right-atrium junction) [5].

The foramen ovale is also a flaplike structure between the right and left atria that acts like a one-way valve. Blood flows across the foramen ovale because high pulmonary vascular resistance maintains pressure in the right atrium at a level greater than that of the left atrium.

The superior vena cava drains deoxygenated blood from the head and upper extremities into the right atrium, where it mixes with oxygenated blood from the placenta.

This blood enters the right ventricle and pulmonary artery where, again, increased resistance in the pulmonary vessels causes 90 % of this blood to be shunted across the ductus arteriosus and into the aorta. This mixture of oxygenated and deoxygenated blood continues through the descending aorta and eventually drains back to the placenta through the umbilical arteries.

The remaining 10 % the blood coming from the right ventricle perfuses the lung tissue to meet metabolic needs. The blood that actually reaches the lungs represents about 8 % of the fetal cardiac output [10,4]. After 30 weeks of gestation the amount of blood perfusing the lungs gradually increases in preparation for birth [5].

During fetal life, the placenta is an organ of low vascular resistance. Clamping the umbilical cord at birth eliminates the placenta as a reservoir for blood, causing a rise in blood pressure and SVR.

As oxygen enters the lungs, the pulmonary vascular bed dilates, increasing blood flow to the lungs and causing pressure in the right atrium to fall. The increased pulmonary venous return to the left atrium and less blood flow into the right atrium cause the left atrial pressure to exceed the pressure in the right atrium, resulting in functional closure of the foramen ovale [12].

After closure, blood is directed from the right atrium to the right ventricle and on to the lungs rather than through the foramen ovale.

Shunting of blood from the pulmonary artery through the ductus arteriosus to the aorta occurs as a result of high PVR. After birth, SVR rises and PVR falls, causing a reversal of blood flow through the ductus and an 8-10 fold increase in pulmonary blood flow [4].

In utero, patency of the ductus arteriosus is maintained by high levels of prostaglandins and the low fetal pO2. Prostaglandins are secreted by the placenta and metabolized in the lungs.

Smaller volumes of blood passing through the fetal lungs result in elevated circulating prostaglandin levels which fall after birth as more blood flows to the lungs [5]. The major contributing factor to closure of the ductus arteriosus is sensitivity to rising arterial oxygen concentrations in the blood [6]. As the pO2(aB) level increases after birth, the ductus arteriosus begins to constrict.

Removal of the placenta decreases prostaglandin levels, further influencing closure [13,1].

Constriction of the ductus arteriosus is a gradual process, permitting bidirectional shunting of blood after birth. PVR may be higher than the SVR, allowing some degree of right-to-left shunting, until the SVR rises above PVR and blood flow is directed left to right.

Most neonates have a patent ductus arteriosus in the first 8 hours of life with spontaneous closure occurring in 42 % at 24 hours of age, in 90 % at 48 hours of age and in almost all infants at 96 hours [9,12].

Permanent anatomic closure of the ductus arteriosus occurs within 3 weeks to 3 months after birth.

Prior to birth the pulmonary blood vessels have a thick layer of smooth muscle, which plays a key role in pulmonary vasoconstriction. After birth this muscle layer becomes less sensitive to changes in oxygenation and begins to thin, a process which continues for 6-8 weeks [17].

Any clinical situation that causes hypoxia, with pulmonary vasoconstriction and subsequent increased PVR, potentiates right-to-left shunting across the ductus arteriosus and foramen ovale [18].

When the umbilical cord is clamped, blood flow through the umbilical vein to the ductus venosus ceases. Systemic venous blood flow is then directed through the portal system for hepatic circulation. Umbilical vessels constrict, with functional closure occurring immediately. Fibrous infiltration leads to anatomic closure in the first week of life in term infants [1].

Respiratory adaptations

At birth the clamping of the umbilical cord signals the end of the flow of oxygenated blood from the placenta. To establish effective ventilation and tissue oxygenation, the neonate must clear the lungs of fetal lung fluid, establish a regular pattern of breathing and match pulmonary perfusion to ventilation.

Other factors, including pulmonary blood flow, surfactant production and respiratory musculature also influence respiratory adaptation to extrauterine life.

In utero, the lung epithelium secretes fluid, a process which is essential to the normal growth and development of the alveoli [22]. Toward the end of gestation, the production of lung fluid gradually diminishes and absorption of fluid begins.

A complete understanding of this process is still lacking but some theories have emerged from work on fetal lambs suggesting that sodium reabsorption plays a key role [11].

The catecholamine surge that occurs just before the onset of labor has also been shown to correspond to a more rapid drop in fetal lung fluid levels [16,19]. Those infants who do not experience labor, such as those born by elective cesarean section, are more likely to have residual fluid in the lungs and develop Transient Tachypnea of the Newborn (TTN) because of lower levels of serum catecholamine [11].

Initiation of breathing is a complex process that involves the interplay of biochemical, neural and mechanical factors, some of which have yet to be clearly identified [1].

A number of factors have been implicated in the initiation of postnatal breathing: decreased oxygen concentration, increased carbon dioxide concentration and a decrease in pH, all of which may stimulate fetal aortic and carotid chemoreceptors, triggering the respiratory center in the medulla to initiate respiration.

Some researchers have questioned the influence of these factors and suggest instead that factors secreted by the placenta may inhibit breathing, and that regular breathing is initiated with the clamping of the cord [1].

Mechanical compression of the chest creates negative pressure and drawing air into the lungs as the lungs re-expand. Further expansion and distribution of air throughout the alveoli occur when the newborn cries.

Crying creates a positive intrathoracic pressure that keeps alveoli open and forces the remaining fetal lung fluid into pulmonary capillaries and the lymphatic circulation.

Thermal and metabolic adaptation

The core temperature of the fetus is typically about 0.5 °C above that of the mother and therefore the fetus expends no energy staying warm [20].

After birth the newborn’s ability to maintain temperature control is determined both by environmental factors and internal physiological processes. Newborns are predisposed to heat loss because of factors such as: a large surface area in relation to body weight, limited body fat and a decreased ability to shiver [2].

Newborns attempt to stay warm by increasing muscle activity and by burning brown fat (non-shivering thermogenesis), which increases metabolic rate. Peripheral vasoconstriction also decreases heat loss to the skin surface.

The production of heat requires oxygen and glucose and produces lactic acid therefore persistent hypothermia may result in metabolic acidosis, hypoglycemia, decreased surfactant production, and over the longer term, poor growth [2].

Maternal glucose readily crosses the placenta and, under normal circumstances, supplies the fetus with enough energy to grow appropriately and to store glycogen in the liver for use after birth.

The release of catecholamines occurring during labor and birth mobilizes glycogen however, blood glucose levels decline after birth, reaching their lowest point at 1 hour of age [21].

NORMAL TRANSITIONAL FINDINGS

Much of the work of transition is accomplished in the first 4-6 hours following delivery, while final completion of the cardiovascular changes may take up to 6 weeks [5].

During the initial hours after birth, the majority of fetal lung fluid is reabsorbed, a normal functional residual capacity is established in the lungs and the cardiovascular system redistributes blood flow to the lungs and tissues. The infant moves through a fairly predicable series of events mediated by the sympathetic nervous system that results in changes in heart rate, respirations, gastrointestinal function and body temperature.

In a classic description still used today, Desmond and colleagues [7] organized these changes into three stages:

  • The first period of reactivity (0-30 minutes) characterized by an increase in heart rate, irregular respirations and fine crackles in the chest with grunting and nasal flaring
  • A period of decreased responsiveness (30 minutes to 3 hours) with rapid shallow respirations, lower heart rate, decreased muscle activity interspersed with jerks and twitches and sleep
  • A second period of reactivity (2-8 hours) in which exaggerated responsiveness, tachycardia, labile heart rate, abrupt changes in tone and color, and gagging and vomiting are commonly seen [7]

Residual symptoms of transition such as crackles in the lungs, a soft cardiac murmur and acrocyanosis may persist for periods of up to 24 hours in otherwise healthy infants [8].

SUMMARY

The majority of newborns complete the process of transition with little or no delay. These infants may demonstrate normal transitional findings, including tachypnea and tachycardia, a soft heart murmur and fine crackles in the lungs as well as acrocyanosis for varying lengths of time after birth.

Prolonged or exaggerated signs of distress should lead the healthcare provider to carry out a thorough physical examination and review of the maternal and newborn history to establish the etiology of the symptoms.

This will allow the rapid initiation of appropriate interventions aimed at minimizing the morbidity resulting from problems of transition or underlying diseases processes. Part 2 of this series will examine signs and symptoms that may assist the healthcare team identify the infant in need of further evaluation during the period of transition.

  1. Alvaro RE, Rigatto H. Cardiorespiratory adjustments at birth. In: Avery&rsquos neonatology pathophysiology & management of the newborn. 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2005: 285-303.
  2. Askin DF. Complications in the transition from fetal to neonatal life. J Obstet Gynecol Neonatal Nurs 2002 31(3): 318-27.
  3. Bellotti M, Pennati G, De Gasperi C, Battaglia FC, Ferrazzi E. Role of ductus venosus in distribution of umbilical blood flow in human fetuses during second half of pregnancy. Am. J. Physiol Heart Circ Physiol 2000 279(3): H1256-63.
  4. Blackburn ST. Maternal, fetal, & neonatal physiology. A clinical perspective. Philadelphia: Saunders, 2003.
  5. Blackburn S. Placental, fetal and transitional circulation revisited. Journal of Perinatal & Neonatal Nursing 2006 20(4): 290-4.
  6. Clyman RI. Mechanisms regulating closure of the ductus arteriosus. In: Polin RA, Fox WW, Abman SH, eds. Fetal neonatal physiology. Philadelphia: Saunders, 2004.
  7. Desmond MM, Rudolph AJ, Phitaksphraiwan P. The transitional care nursery. A mechanism for preventive medicine in the newborn. Pediatr Clin North Am 1966 Aug 13(3): 651-68.
  8. Gardner SL, Johnson JL. Respiratory diseases. In: Merenstein GB, Gardner SL, eds. Handbook of neonatal intensive care. 6th ed. St. Louis: Mosby Elsevier, 2006: 79-121.
  9. Gentile R, Stevenson G, Dooley T et al. Pulsed Doppler echocardiographic determination of time of ductal closure in normal newborn infants. J Pediatr 1981 98: 443-448.
  10. Grant DA. Ventricular constraint in the fetus and newborn. Can J Cardiol 1999 15 (1): 95-104.
  11. Jain L, Eaton DC. Physiology of fetal lung fluid clearance and the effect of labor. Seminars in Perinatology, 2006 30: 34-43.
  12. Johnson BA, Ades A. Delivery room and early postnatal management of neonates who have prenatally diagnosed congenital heart disease. Clin Perinatol 2005 32: 921-946.
  13. Kenner C. Resuscitation and stabilization of the newborn. In: Kenner C, Lott JW, (eds). Comprehensive neonatal nursing: a physiologic perspective. 3rd ed. Philadelphia: Saunders, 2003: 210-27.
  14. Kiserud T, Acharya G. The fetal circulation. Prenat. Diagn 2004 24(13): 1049-59.
  15. Kiserud T, Rasmussen S, Skulstad S. Blood flow and the degree of shunting through the ductus venosus in the human fetus..Am J Obstet Gynecol 2000 182(1 Pt 1): 147-53.
  16. Lagercrantz H, Bistoletti P. Catecholamine release in the newborn infant at birth. Pediatr Res 1977 11(8): 889-93.
  17. Lakshminrusimha S, Steinhorn RH. Pulmonary vascular biology during neonatal transition. Clin Perinatol 1999 26(3): 601-19.
  18. Lott JW. Assessment and management of the cardiovascular system. In: Kenner C, Lott JW, eds. Comprehensive neonatal nursing: a physiologic perspective 3rd ed. Philadelphia: W.B. Saunders, 2003: 376-408.
  19. Pfister RE, Ramsden CA, Neil HL et al. Volume and secretion rate of lung liquid in the final days of gestation and labour in the fetal sheep. J Physiol 2001: 535(3): 889-99.
  20. Rutter N, Hull D. Response of term babies to a warm environment. Arch Dis Child 1979 54(3): 178-83.
  21. Sinha SK, Donn SM. Fetal-to-neonatal maladaptation. Semin Fetal Neonatal Med 2006 11(3): 166-73.
  22. Strang LB. Fetal lung fluid: secretion and reabsorption. Physiol Rev 1991 71: 991-1016.
  1. Alvaro RE, Rigatto H. Cardiorespiratory adjustments at birth. In: Avery&rsquos neonatology pathophysiology & management of the newborn. 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2005: 285-303.
  2. Askin DF. Complications in the transition from fetal to neonatal life. J Obstet Gynecol Neonatal Nurs 2002 31(3): 318-27.
  3. Bellotti M, Pennati G, De Gasperi C, Battaglia FC, Ferrazzi E. Role of ductus venosus in distribution of umbilical blood flow in human fetuses during second half of pregnancy. Am. J. Physiol Heart Circ Physiol 2000 279(3): H1256-63.
  4. Blackburn ST. Maternal, fetal, & neonatal physiology. A clinical perspective. Philadelphia: Saunders, 2003.
  5. Blackburn S. Placental, fetal and transitional circulation revisited. Journal of Perinatal & Neonatal Nursing 2006 20(4): 290-4.
  6. Clyman RI. Mechanisms regulating closure of the ductus arteriosus. In: Polin RA, Fox WW, Abman SH, eds. Fetal neonatal physiology. Philadelphia: Saunders, 2004.
  7. Desmond MM, Rudolph AJ, Phitaksphraiwan P. The transitional care nursery. A mechanism for preventive medicine in the newborn. Pediatr Clin North Am 1966 Aug 13(3): 651-68.
  8. Gardner SL, Johnson JL. Respiratory diseases. In: Merenstein GB, Gardner SL, eds. Handbook of neonatal intensive care. 6th ed. St. Louis: Mosby Elsevier, 2006: 79-121.
  9. Gentile R, Stevenson G, Dooley T et al. Pulsed Doppler echocardiographic determination of time of ductal closure in normal newborn infants. J Pediatr 1981 98: 443-448.
  10. Grant DA. Ventricular constraint in the fetus and newborn. Can J Cardiol 1999 15 (1): 95-104.
  11. Jain L, Eaton DC. Physiology of fetal lung fluid clearance and the effect of labor. Seminars in Perinatology, 2006 30: 34-43.
  12. Johnson BA, Ades A. Delivery room and early postnatal management of neonates who have prenatally diagnosed congenital heart disease. Clin Perinatol 2005 32: 921-946.
  13. Kenner C. Resuscitation and stabilization of the newborn. In: Kenner C, Lott JW, (eds). Comprehensive neonatal nursing: a physiologic perspective. 3rd ed. Philadelphia: Saunders, 2003: 210-27.
  14. Kiserud T, Acharya G. The fetal circulation. Prenat. Diagn 2004 24(13): 1049-59.
  15. Kiserud T, Rasmussen S, Skulstad S. Blood flow and the degree of shunting through the ductus venosus in the human fetus..Am J Obstet Gynecol 2000 182(1 Pt 1): 147-53.
  16. Lagercrantz H, Bistoletti P. Catecholamine release in the newborn infant at birth. Pediatr Res 1977 11(8): 889-93.
  17. Lakshminrusimha S, Steinhorn RH. Pulmonary vascular biology during neonatal transition. Clin Perinatol 1999 26(3): 601-19.
  18. Lott JW. Assessment and management of the cardiovascular system. In: Kenner C, Lott JW, eds. Comprehensive neonatal nursing: a physiologic perspective 3rd ed. Philadelphia: W.B. Saunders, 2003: 376-408.
  19. Pfister RE, Ramsden CA, Neil HL et al. Volume and secretion rate of lung liquid in the final days of gestation and labour in the fetal sheep. J Physiol 2001: 535(3): 889-99.
  20. Rutter N, Hull D. Response of term babies to a warm environment. Arch Dis Child 1979 54(3): 178-83.
  21. Sinha SK, Donn SM. Fetal-to-neonatal maladaptation. Semin Fetal Neonatal Med 2006 11(3): 166-73.
  22. Strang LB. Fetal lung fluid: secretion and reabsorption. Physiol Rev 1991 71: 991-1016.

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Neonatal Nurse Practitioner
St. Boniface General Hospital
Winnipeg, Manitoba, Canada

Associate Professor
Faculty of Nursing
University of Manitoba
Winnipeg, Manitoba, Canada

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Watch the video: Patient Education Animation: Labor and Vaginal Birth (July 2022).


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