Bipedalism, big-headed babies make childbirth challenging.
Moms giving birth often have quite an entourage these days. In addition to doctors, nurses and a partner by their side, women sometimes invite in a parent or friend or hire a doula to coach them through the breathing and pushing.
While the cast of characters in the birthing suite has changed over the decades, for humans, birth has mostly been a social affair. Paleoanthropologist Karen Rosenberg, from the University of Delaware, says there’s likely an evolutionary reason. She spoke with WHYY’s “The Pulse.”
So what do you do?
I study human fossils from all over the world as a way to try to understand how humans evolved.
Why study childbirth?
It’s quite different in humans than in other animals. Humans have a more complicated, more difficult, and perhaps riskier time giving birth, and that’s related to two features of our anatomy, namely the fact we are bipedal—walk on two legs—and the fact that we have large brains. The baby’s head really just barely fits. If we had smaller brains we wouldn’t have as high risks during childbirth that we do. Historically, death in childbirth was the leading cause of death for women of childbearing age.
How did childbirth change as humans evolved?
Our pelvis is completely modified from the kind of pelvis that our ape ancestor would have had. The actual birth canal in an ape is a sort of straight tube, whereas in humans, it’s a kind of curved tube, so at the top it’s wide from side to side, then it becomes long from front to back, and then approximately circular. It would be like if you were trying to force an egg through a toilet paper tube, and the tube was smashed to the side at one point, and you would have to twist the egg to make it pass through.
The baby makes rotations as it travels through the birth canal?
Exactly. The shape of our pelvis changed five to seven million years ago in our ancestors, people like Lucy—everybody’s heard of Lucy—the famous Ethiopian fossil from three–three and a half million years ago. So, that completely modified the pelvis, and that was for reasons that had to do with walking. Then, later in our evolutionary history—starting between two and three million years ago—we start to get large brains, and when the brains got big further modifications in the pelvis had to be made, and further modifications in the mechanism of birth.
Other mammals often go off by themselves to give birth, why’s that?
She’s hiding from predators, she’s hiding from interfering members of her own species, and humans do just the opposite of that. When humans give birth they look for humans to be around. We find our sisters, our husband, our mothers, and our midwives or obstetricians. We are looking to have people there to help us.
Your theory includes the idea that anatomy has moved us to call in help when we are giving birth?
We think that women benefit from having assistants for a bunch of reasons. [For example,] clearing the baby’s breathing passage, by lifting the baby out of the birth canal without injuring its neck or its spinal cord. A monkey mother can lift the baby up to her breast, and monkey babies can help themselves up to the breast. They’re strong, and monkey babies have something to cling to, which is that their mothers have fur.
Human babies are born in a kind of unusual condition compared to our close relatives. Ashley Montagu, a 20th-Century anthropologist, used to say that humans have “exterogestation.” What he meant is that human babies continue to grow like fetuses even after they’re born. They continue to have a period of really rapid brain growth. They are born at a very early stage of development compared to other animals.
Our babies are helpless but big. So when you think about the attention that human babies suck up from their families, the fact that they need care for a long time, and the fact that they are big things to carry around as people remember is really important, as well.
You have two daughters, what did your own birth stories teach you about all this?
I’m pretty critical of over medicalization of childbirth. I think that the Cesarean-section rates in the United States—and even more so in countries like Brazil and Italy—are extremely high, probably much higher than they should be for the welfare of mothers and babies. Nevertheless, with my first child, she was breach, which meant that instead of being born head first, as is typical—and as is safest—she was being born bottom first, and so the doctors said that they wanted to do a Cesarean section. There is a risk of brain damage in vaginal breach births. I was very happy to have a Cesearean section with her. Then my second daughter was in a so-called normal position, I had a very easy, natural childbirth with her.
How might your theories inform birth in the 21st Century?
Thinking about seeking assistance is an important consequence of the constraints that are imposed on us by our biology. Looking at childbirth in an evolutionary context gives us some insight into how to practice modern obstetrics or modern midwifery because it encourages us to understand that the way women behave during childbirth makes evolutionarily sense. I think that when we put women in labor in a really clinical, strange situation where they don’t have emotional and social support, they don’t do as well. There’s really good evidence that they have higher rates of intervention. Not to say that medical intervention is never necessary—of course it is.
Editor’s note: This Q&A is shortened and edited from the original.