Why Is Childbirth So Difficult? | Darwin, MD #1
imagine serving as a physician in the obstetrics and gynecology department of a large Hospital two women arrive on the same day ready to each give birth to their first child both are 25 and experienced an uneventful pregnancy they are fit and ready for the experience both planned for a vaginal delivery one did give birth vaginally but the other ultimately needed a cesarean section what happened why would two healthy women of similar size weight and fitness experience different childbirth outcomes my name is Florence UN and this is Darwin MD it's no secret that giving birth is hard each day some 830 women die from preventable causes related to pregnancy and childbirth the global mortality rate is 216 deaths per 100,000 live births meanwhile the global infant mortality rate is trending downward the United Nations Department of Economic and Social Affairs reports 35 deaths per 1,000 live births between 2010 and 2015 that's down from 142 deaths per 1,000 live births in the 1950s why is it so difficult for humans to give birth this seems to run counter to evolution an efficiency seeking process designed to make sure our genes pass smoothly between generations our closest relatives the great apes have a relatively easy time with giving birth and will generally give birth alone to avoid infection or stress from others for humans though it takes a village mothers will actively seek out others to help during childbirth this illustrates the concept of trade-offs discussed in the first episode at some point in human evolution the advantages of human company outweighed the disadvantages of having other humans and all their bacteria around nevertheless childbirth is still difficult even with lots of help returning to the trade-off model making childbirth easier may actually work against gene transmission from generation to generation the problem which evolutionary medicine researchers are still arguing about today is what trade-off makes human childbirth so difficult now there are probably many trade-offs which all build upon each other to make childbirth hard we'll get into that more later but when scientists first started thinking about this in the mid 20th century they developed an explanation to neatly justify why childbirth is hard that medical intervention would be a necessary part of the process to save the lives of mothers and children first let's discuss the factors that make human development unique among primates human babies are less developed than the babies of most of our close relatives in several interesting ways most primates are precocial that is babies are born with some degree of independence and mobility able to open their eyes and hold on to their mothers soon after birth on the other hand human babies are far more altricial than our primate relatives for example infants motor skills are incredibly immature try getting a newborn human to hang from its mother the way baby chimpanzees do brain development likewise needs time the brains of baby chimpanzees are 40 percent of adult size humans not so much infant brains are less than a third of the size of adult brains based on these characteristics of human development scientists thought of an explanation which became and arguably still is the prevailing explanation for why childbirth is hard it's called the obstetric 'old ulema the theory goes like this hominins the evolutionary ancestors of humans after they split from other primates began to walk upright on two legs known as bipedalism this caused a variety of changes to take place in the pelvis and lower limbs most notably the pelvis became narrower as muscle groups changed their structure and position the birth canal also changed in shape so that the entrance into the pelvic cavity was oriented differently than the exit because of this babies couldn't just slide straight out of the birth canal their heads and shoulders had to twist inside the pelvis so that they could exit through the widest parts of the birth canal after hominins became bipedal a second challenge presented itself hominins brain started growing in size and complexity in response to new environments and other factors a process known as encephalization this meant that newborns brains were also larger and these larger brains were enclosed by larger skulls which needed to fit through female hominins increasingly narrow pelvises now we have all of the ingredients for a neat and tidy explanation childbirth is hard the obstetric will dilemma says because widening the pelvis to make birth easier would make it impossible for female humans to walk properly on two legs so babies are born heads and brains that only just fit through the females narrow pelvis problems arise when the baby's head is too big to fit through a complication known as cephalo pelvic disproportion as for why human babies are more all trishal than those of other primates the obstetric will dilemma says that babies are born developmentally premature so that their heads are smaller and more likely to fit through the birth canal since the 1960s when the obstetric L dilemma hypothesis first emerged it's been generally accepted as the explanation for why childbirth is hard however it is often the case that the truth resists simplicity recent research has called into question some of the fundamental assumptions of the obstetric dilemma making our job of answering the question why as child births are difficult much more well difficult for example do we actually give birth earlier and to more premature babies than other primates it turns out that no we don't human pregnancies are both absolutely and relatively longer than those of chimpanzees gorillas and orangutans in fact in a 2012 paper Halle dunnsworth and colleagues calculated that human gestation length is 37 days longer than expected for a primate of similar body mass compared to other primates we also give birth to newborns weighing 50% more than expected and these newborns brains and bodies are huge compared to the body size of their mothers this means that human babies are not born prematurely and in fact human mothers may devote more of their resources to their babies than other primate mothers do what about the pelvis there's no denying that it's a tight fit to get babies through the narrow birth canal but is the female pelvis uniquely restricted because of bipedalism not necessarily a 2015 paper by Anna Warriner and colleagues used a dynamic model of hip mechanics to compare the efficiency of walking and running in male and female subjects they found that a wider pelvis did not lead to less efficient walking or running technically humans could have even wider hips to make childbirth easier without compromising their ability to walk but for whatever reason there hasn't been a need to evolve wider hips this means that the need to walk on two legs is not what's restricting the size of the pelvis as the obstetric will dilemma suggests additionally the problem of a tight fit or cephalo pelvic disproportion isn't even unique to humans some primates with smaller bodies such as monkeys and Gibbons also have a close correspondence between the size of the birth canal and the size of the baby's head cephalo pelvic disproportion does sometimes occur if this problem is due to the constraints of bipedalism then why does it also exist in monkeys and Gibbons they don't walk on two legs like we do plus if the female pelvis really is so tightly constrained by the trade-off between bipedalism and encephalization then female pelvis should be similar in shape that is stabilizing selection should be acting on the female pelvis to keep that delicate balance between upright walking and large brains however that hasn't been the case either a 2015 study found that the human female birth canal varies even more in size and shape than human limbs do so our arms our arms may be more similar in size and shape than our pelvises obviously our arms aren't under the constraints of this trade-off these recent papers are part of the growing body of evidence that while the obstetric will dilemma sounds neat and simple it is probably not the best explanation for why childbirth is hard how can these contradictions be resolved Halle dunnsworth's 2012 paper offers an alternative hypothesis which she calls the energetics of gestation and growth or egg hypothesis this hypothesis suggests the pregnancy ends when the energy needs of the growing fetus exceed the mothers ability to meet those needs the basal metabolic rates of pregnant mothers quickly increased to near the upper limit for human metabolism as the fetus develops and grows bigger if pregnancy was even one month longer the mother would probably become unable to provide enough resources to support both herself and the fetus so the trade-off that makes human childbirth so difficult may not be the balance between bipedalism and encephalization as the obstetrical dilemma hypothesis states rather the trade-off maybe between fetal energy needs and maternal energy supply as it is in primates and other mammals here's the thing though like we touched on earlier the truth resists simplicity it's likely that both of these trade-offs and more are at play in the complicated process that is childbirth for example nutrition likely plays a crucial role in the size of both mother and baby which would play into both of the trade-offs we've discussed there is also recent evidence that the female pelvis actually changes shape during puberty to help facilitate childbirth and reverts back during menopause when childbirth is no longer a concern which further complicates the discussion the debate over what makes childbirth hard is a prime example of how science and medicine can change our understanding of ourselves and the world with the advent of cesarean sections also known as these sections and other medical technologies the role of the pelvis and human childbirth is changing more babies are born alive today than in the past due to evolutionary constraints overcome by medical intervention who knows how that might change our evolutionary future remember the two women who opened the episode let's determine why they took different approaches to birthing the obstetric 'old ulema would drive conclusion that the woman with the c-section probably had a birth canal too small for her baby's head to squeeze through a constraint caused by her need to walk on two legs the conclusion would be different under the egg hypothesis the baby powered by the mothers ample investment in resources grew too large for a normal birth canal what can be both frustrating and fascinating about evolutionary medicine questions is that often there is no clear-cut answer perhaps the answer to this episode's Y question is a combination of both explanations or includes other factors that we haven't even talked about for example the woman who needed a c-section may have had a condition called placenta previa where the placenta partially or completely covers the cervix which is where the baby would exit the uterus this makes a c-section essentially required for a pregnancy which might otherwise be completely healthy ultimately as in the cases of both cephalo public disproportion or placenta previa there are circumstances where a caesarean section is required to preserve the life and health of both mother and baby when do you need a c-section though and when would a vaginal delivery be better and no matter how you're giving birth what kind of personal and medical support provides the biggest benefits and what is unnecessary or even harmful that'll be the topic of our next episode of Darwin MD see you then you
