Why is human childbirth so much longer and more painful than that of other primates? For more than 60 years, a popular hypothesis known as “the obstetrical dilemma” has sought to explain our species’ difficult childbirth by arguing that evolution has stuck women with a pelvis that’s ideal for walking upright but too narrow to easily birth our large-brained offspring.

Biological anthropologist Holly Dunsworth begs to differ. Writing in “The Evolution of Difficult Childbirth and Helpless Hominin Infants” in the Annual Review of Anthropology, she and her coauthor Leah Eccleston argue that science no longer supports the obstetrical hypothesis, and that it’s time for new thinking. Women’s bodies, they say, are exquisitely adapted to continue the species.

This interview has been edited for length and clarity.

What makes human childbirth so difficult?

It’s much more complicated than simply big baby, little hole. We are not actually unique in having a tight fit between the size of the newborn and the birth canal. Certain kinds of monkeys, like macaques and marmosets, have tight fits, too, and they aren’t known for having the difficulty in birth that many humans do.

What is unique to humans is that our pelvis changed as we evolved habitual bipedalism. For humans, some of the reason for the tight fit is that our babies have to get through a twisted tube, while other primates just have a ring or a hoop. Our kids have to get through a couple of hoops tied together in different dimensions. This is why it’s helpful if babies twist as they are coming out. Our birth canal is also surrounded by more bone than those of other primates, which are fleshier.

This is the pelvis we got from evolution and it’s been great and worked out pretty well for bipedalism, even though it makes childbirth trickier. But the incidence of an impossibly tight fit, called cephalopelvic disproportion, is very low, actually.

We are bad at judging the size of babies in utero. I have studies of this open on my computer right now that show that overestimation of babies’ weights by ultrasound leads to a higher risk of going straight to cesareans, even when they may not be necessary. Obstetricians have very good intentions and they are trying to mitigate risk, which is a noble cause. But it is not so simple to measure Mom’s pelvis or Baby’s head.

Then where did the idea that a tight fit causes human births to be long and painful come from?

In the 1950s, some doctors actually tried to measure Mom’s pelvis using X-rays, (they didn’t know back then that this posed unnecessary risks for both the mother and fetus). This is when the “obstetrical dilemma” hypothesis first arrived — the idea that as our babies got bigger-brained with evolutionary selection, our birth canals couldn’t expand along with that, because selection for efficient bipedalism required a narrower pelvis.

One evolutionary “solution” to the dilemma that has been proposed is that humans give birth to our babies early, so they are born in time to escape. But that is fundamentally flawed, because we are not born early — we have the longest gestation of all the primates. Also, no one has shown that our hips are at the maximum possible width that would enable bipedalism — that the birth canal cannot expand more during evolution. It has been theoretically argued, but not experimentally shown.

I’m not sure how testable the obstetrical dilemma hypothesis is. It’s based on a lot of assumptions about women’s bodies that we’ve grown out of, academically and scientifically. We now know that women’s hips are not the reason why men hold the world records for walking and running competitions. And, in general, women’s bipedalism is equal in competence to men’s. And women are capable of giving birth to really big babies.

Illustration of the female pelvis, from a book on pregnancy and childbirth by French obstetrician François Mauriceau, “Traité des maladies des femmes grosses, et de celles qui sont nouvellement accouchées,” published in 1675.

CREDIT: WELLCOME COLLECTION

Are we really saying that women’s bodies are an evolutionary dilemma? How much of a dilemma could it be for us to have gotten to the point of 7 billion people? We got to that number largely without cesareans. All things being equal, I think we are underestimating the capabilities of women’s bodies.

We could talk for hours about all the factors that lead to difficult childbirth. We don’t necessarily labor in positions that allow our pelvic bones to expand at the sutures to expand the birth canal a little bit. Do we experience more pain than other primates? It’s possible. We have the biggest-brained and largest babies of all primates. But have we always had huge babies? Through lifestyle and diet we probably have a lot of energy today to handle the growth of a huge fetus.

What can the fossil record tell us about difficult childbirth?

We have a few fossils that are complete, or complete enough, from Australopithecus times, 4 million years ago. [Editor’s note: Australopithecus is a group of hominins that are considered to be the first to walk upright and are the ancestors to the Homo group, from which modern humans evolved.]  We can see that they have more of a constrained, bony birth canal for the fetus to push through, which evolved alongside bipedalism.

The 3.2-million-year-old fossil Lucy has one of the earliest, most complete pelvises of any hominin fossil. From Australopithecus skeletons, we can reconstruct adult brain size, and because adult brain size correlates closely with newborn brain size across hominins, from that we can estimate how big Lucy’s baby’s brain and birth weight would be. With a lot of assumptions, Jerry DeSilva of Dartmouth College did that to estimate that Australopithecus had these tight fits. But we don’t actually know that. Did they have a tight fit or did they have smaller babies, like chimps?

When do you think hominin childbirth first became difficult?

I don’t know. We can’t identify fossil evidence of women dying in childbirth. There are archaeological remains of pregnant women with full-term fetuses, but we cannot tell if they died in childbirth or some other way.

I don’t think the fossil record is where to learn about human childbirth. I think studying the live pregnancies of humans and other primates will be more informative.

You say you don’t agree with the hypothesis that our babies are born early to escape an impossibly tight fit. What else might determine the timing of birth?

We are probably born according to the same fundamental physiological processes going on in other primates and placental mammals. What can a mother sustain metabolically day after day? She doesn’t just eat for the fetus, but breathes for the fetus, too. How long can a mammal sustain that elevated metabolic rate?

We have put forth a different hypothesis: the energetics of gestation and fetal growth, or EGG, hypothesis. This says that birth is initiated at the critical point when the mother can no longer metabolically support the growing fetus. We plotted the mother’s additional energy required against the energy required by the growing fetus across the 9.5 months of gestation. We showed that women are operating at full throttle in the last months of pregnancy. Then the needs of the fetus outstrip that, and Mom hits a ceiling of a maximum sustainable metabolic rate that is about two times her basal metabolic rate. It seems to explain the timing of birth in humans and I’m hoping to find out whether it explains gestation length in other species, too.

That might explain why a chimpanzee gives birth to a relatively much smaller newborn than we do. The baby chimp is born before there’s a size problem — so what makes it be born, then? Maybe it’s the cost of the energy going to the fetus.

How would you test this idea?

We are studying the energy use and metabolic changes in pregnant and lactating marmoset monkeys to see how their pattern compares to humans. Ideally, I want to study this in pregnant and lactating apes — chimpanzees, bonobos, gorillas, orangutans and gibbons — to see if they, too, have that sustained maximum metabolic rate that plateaus in late pregnancy in human beings. But I’m doing this right now in marmosets, because they are tiny monkeys and less expensive.

Differences in metabolic physiology aside, our modern lives are also different from other primates’. So many lucky women have virtually unlimited access to calories when pregnant. You can sit on the couch and eat Ben & Jerry’s ice cream for breakfast, lunch and dinner. That’s not the lifestyle of a pregnant orangutan, even at a very good zoo.

Why is it important to trace the evolution of childbirth?

On one level, it’s inherently fascinating to me. Human evolution and reproduction and sex are such a huge part of the story of where we came from.

Also, it’s important for me as a woman. I was told as a girl that I couldn’t play soccer with the boys because I could injure my uterus and then I might not be able to have babies when I grew up. I sense that a lot of the science that contributed to these ideas has lingered well beyond its shelf life. Women are not the lesser-evolved body and I can’t help but want to buck that with science and scholarship.

So I might have an ax to grind, but in a good way. It’s energizing. I would like to lift the burden of the obstetrical dilemma hypothesis from obstetric medicine and midwifery and every woman’s mind. It encourages the underestimation of women’s bodies and the overestimation of risk from childbirth, and along with those, unnecessary childbirth interventions.

You write “females are the gateway to continued evolution.” What do you mean by that?

With our bony birth canal, women literally decide if the lineage goes extinct. That it hasn’t reflects how well we are adapted for continuing the species, even with difficult childbirth.

Some of the past thinking on this is attempting to explain what is “wrong” with women’s bodies. I think that is the glass-half-empty view of evolution. In the glass-half-full view, we can view women’s bodies as clearly well adapted.