Science Friday - Some primates undergo even more extreme childbirth than humans

Episode Date: August 11, 2026

Human childbirth is often difficult and incredibly painful—just ask anyone who’s been through it. And for a long time, childbirth seemed to be uniquely difficult for humans. But a new study suggests that we aren’t alone: Some small primates birth offspring whose heads are nearly twice the size of the mother’s pelvic area.   So how do those primates go through difficult childbirths? Why didn’t evolution make ours any easier? And what can this tell us about humankind? Flora explores these questions and more with anthropologist Lia Betti. Guest: Dr. Lia Betti is an associate professor of anthropology at University College London in the UK. Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Follow our show on Instagram, TikTok, Facebook, and Bluesky @scifri and sign up for our newsletters. Got a science question that’s keeping you up at night? Call us: 877-472-4374 Hosted by Simplecast, an AdsWizz company. See pcm.adswizz.com for information about our collection and use of personal data for advertising.

Hey, it's Flora and you're listening to Science Friday. Childbirth is often difficult and painful. Just ask anyone who's been through it. That is not news. But why is it that way? And is our experience unusual compared to our primate relatives? You know, what can anthropology and evolutionary biology tell us about our own birthing experience? That's what my next guest studies.

And in her new research paper, she reports that we aren't alone when when it comes to extreme, even dangerous childbirth. Some other species bear it, too. Here to chat is Dr. Leah Beti, anthropologist at the University College London. Leah, welcome to Science Friday. Pleased to be here. Okay, so why is human childbirth so hard? Is it a geometry problem?

That's the interpretation, the learning interpretation, that we have a very difficult situation just because of the way we evolved. So we are bipedal, we walk on two legs, and that meant that our pelvis had to adapt to it and change in shape and size, and we ended up with quite a compact little space for the baby to be born. However, at the same time, we had this very big brain that evolved, that is massively larger than the ones of other primates. So we ended up in this unusual situation of having to fit a particularly large brain,

out of a particularly small pelvis. And that's a history that has been told for, well, at least since 1960, when the term obstetrical dilemma was invented just for us. The obstetrical dilemma. Yes. Such a very scientific way of putting it. How does that compare to other species?

Where are we on the spectrum? That's exactly the question. So in the past, we were compared with other big apes, like the chimpanzees, also gorillas, also gorillas and orangutans, and they're all quite big apes, and they have a relatively small babies. In their pelvis, the baby can go through quite easily. So it looked like we were the very unlucky one. But in this study that we have done, we found that in other small primates, in some primates, the situation is so much worse. So there are these little monkeys called squirrel monkeys. They're about half a kilo more or less.

and they give birth to pretty big babies up in the canopy in the forest. And when the baby's head has to go through the pelvis, the baby's head is actually double the size of the space available in the pelvis. So it looks like it totally is impossible to fit the baby's head through the pelvis. However, clearly they manage because they do give birth regularly. So what seems to be happening there is that the maternal pelvis totally, totally dislocated. The three bones that form the pelvis completely separate during birth in order to allow this huge head of a baby to go through, which of course doesn't happen.

Humans. We cannot dislocate our bones. Wow. Okay. So in these squirrel monkeys, the baby's head is double the size of the pelvis? Yes, and they are not alone. There are other primates that have such an impossible situation. So in perspective, our birth should on paper be, relatively easy. Well, give me some examples of these other impossible situations and how they cope, because I guess not everybody's dislocating all of their pelvic bones. Well, the thing is, we don't really know. So we have other little monkeys called tamarines, and they also have a baby's head that is almost double the size of the maternal pelvis. But nobody has actually observed what happens during birth. So we don't know how they manage. We just assume that they're

do the same as the squirrel monkeys. But again, nobody could observe this. We know what happens for squirrel monkeys because what I think is a heroic student took x-rays of the birth of squirrel monkeys in the 90s, so sat with the monkeys throughout their labor and during birth and took x-rays in the moment when the mother was pushing. So she could actually see in the X-rays. So she could actually see in the x-ray that the pelvis was dislocating, that the bones were completely separate in order to let the baby through. But, of course, we don't have this for the majority of the other species. Wow. That's amazing. Do we know if childbirths are painful for other primates? Can we know? That's the question. It's so hard because when women give birth, and it depends on the population,

but women tend to vocalize, to make noises quite a lot. Other primates do not do that. We don't know why. Maybe it's because living in the wild with the predators nearby, vocalizing is not a good idea. However, I've had to watch quite a few videos of primates giving birth, and you can see the tension in the body every time there is a contraction. So I can only assume that it is a difficult.

and painful experience. What we know, though, it's shorter than for humans. So we do seem to have a longer birth, longer labor. So maybe we do have a difficult situation somehow, but it's not down to how big is the baby in respect to the pelvis. Something else is going on. What?

Okay, here's my question for you. Why would childbirth be painful and hard? I mean, I'm speaking from experience too, but evolutionarily, this seems like a bad idea. I totally hear you. and also speaking some experience, it was not fun at all. And it also is quite dangerous. I mean, now we have cesarean sections and hospital care and also the epidural if we want to.

But in the past, none of this was available. So you can expect that maternal mortality and infant deaths as well was quite higher than it was today. And we have records of that. So it was painful and difficult and dangerous. Why is that the case? So the obstetrical dilemma that we talked about, the big hypothesis from the 60s, was all about how big was the baby's head in respect to the maternal pelvis. But now we know that actually we don't have it as badly as other primates. So it might be also that we have very fat babies. I mean, honestly, our babies just deposit huge amounts of fat. That's not the case in other primates. So there was a study recently that says, that if you imagine that it's a good thing to have a big baby, because the bigger the baby, the easier is for the baby to survive after birth. So we have a pressure evolutionarily to have big babies.

At the same time, we have adilatively-compart pelvis to be able to walk on two legs. In this situation in which you have two opposing needs, what you can expect in nature, based on normal evolution, theory is that there will be some deaths, effectively, because we are combining two opposing needs. Hmm. What adaptations do humans have for giving birth? I mean, like, should we think of adaptations around support systems, like midwives? Well, exactly.

So we are stuck in this situation, having a difficult birth, but evolutionarily, Some of the ways in which we adopted is cultural. In other primates, you don't have midwives. Usually, the mother gives birth on her own, sometimes in complete isolation, or even if she is in a group, she will not have direct support. So humans seems to have fairly universally evolved midwifery, and that seems to be fundamental. And one of the reasons why it might be so important for us,

is because our babies come through the birth canal and emerge facing backwards, so the back of the mother. So the mother herself cannot easily help the baby out by pulling the baby because it will pull against the direction of the spine of the baby, because the baby is facing the other way. While in other primates, usually their babies come out looking the front of the mother, and the mother can help the baby out herself. So we cannot do that, hence probably we do have midwives.

Now, what is interesting is that in order to have midwives, you need to have quite complex cognition and a complex social system. So it is possible that our difficulties in giving birth might have been also a stimulus to evolve a more complex social system and more complex cognition and support for the mother. Hmm. That's fascinating. Did this work make you rethink any of your own preconceived notions about giving birth? Yes. I mean, I thought we were in the worst possible situation, especially after personal experience. And now I do feel a twinge of regret that I didn't know this beforehand. I went in more level-headed. Yes, no, I do feel pity for some of the other monkeys, I have to say. They can't even express how. Happy they are. Is there a story?

You looked at, you know, dozens of species. Is there a story, an extreme birth story, that really sticks out to you as one that you'll never forget? Oh, gosh. I've seen so many videos. There is one photo that sticks in my mind of a gelada monkey mother giving birth. And the baby is facing out of the vagina with their face forward and looks at you. as it emerges, which I find is absolutely hilarious that the baby just comes through looking at the world.

It's very, very different from what we have. But yes, there are other stories that are more complex. For example, I cannot imagine how terrifying must be for the mother to give birth up in the trees as it happens in quite a few of the small monkeys that live quite high up from the ground. and they do give birth hanging from branches or sitting on branches and in some cases the baby falls and it's only held up by the umbilical canal

so it's attached to the mother and the mother needs to fish it up and I'm assuming there are unhappy stories there but it must be absolutely terrifying oh my goodness like a yo-yo, just like pulling it back up. I wasn't going to say that, but yes, like a yo-yo, probably. Yes. Dr. Leah Betze is an anthropologist at the University College London. Leah, thank you for joining me today. Thank you for inviting me.

This episode was produced by Rasha Auretti. We will catch you next time. I'm Flora Lichten.