Perhaps my interest in studying the evolution of the human pelvis had something to do with a feeling that my pelvis had just betrayed me during childbirth. My first pregnancy had been uneventful, at least according to doctors, until the emergency cesarean section that brought my son into the world. A year later, as a second-year graduate student looking for a new thesis project, I heard a lecture on human locomotor efficiency that lit up a dormant idea I had assimilated long ago. “Isn’t it true,” I said to the presenter afterwards, “that women are less efficient walking and running than men because our pelvises have been selected for childbirth?”
The obstetrical dilemma, a term first coined by Sherwood Washburn in 1960, describes what he saw as the competing demands on the female pelvis of bipedalism and birthing big-brained infants. The paradigm has since expanded beyond its original meaning to explain many human phenomena, including maternal mortality, birth timing, infant development, and even aspects of social organization. But as I was to discover when I began testing the relationship between pelvic shape and locomotor efficiency, the obstetrical dilemma (at least as originally conceived) had been built on a shaky foundation.
There is no doubt that human childbirth can be difficult and even dangerous―maternal morbidity and mortality are serious problems globally―but a hyperfocus on the obstetrical dilemma as the key explanation masks more insidious culprits that sometimes place mother and baby at risk.
Evolution of the pelvis and big-brained babies
When human ancestors began moving terrestrially on two feet instead of four, that change in locomotor pattern necessitated a fundamental repatterning of pelvic structure. The top, blade-like bones, or ilia, broadened and rotated into a position on the sides of the body. The pelvis got shorter and more robust, and portions of the birth canal became encircled by a bony ring. These changes were necessary for the mechanics of bipedalism, particularly the repositioning of the ilia which allowed the hip abductors to become the primary stabilizers of the pelvis in the side-to-side direction during walking and running. When these muscles don’t function properly, the pelvis drops away from the standing limb with each step, reminiscent of models on the catwalk. While this movement pattern may be in vogue in the fashion industry, it’s not a particularly efficient way to get around. Each time the pelvis sags, it has to be lifted back up by muscular work which requires energy. So the hip abductors ensure stability and minimize the energetic cost of locomotion (thus thwarting your future modeling career).
Several million years after this profound change in locomotor behavior and the accompanying adaptations of the pelvis, human ancestors started to get bigger brains. The causes of this increase in brain size are multifactorial, but certainly had to do with changes in diet, probably hunting, social behaviors, and maybe selection for increased tool use, as Washburn envisioned.
The process of growing these large adult brains had to start in utero. Human babies are born with 30 percent of their adult brain size, which is relatively less than in other apes. But measures relative to adult brain size skew the analyses since humans have such a large adult brain compared to other apes. A better measure of the comparative size of infant brains is to look relative to maternal body mass. That comparison shows that human mothers give birth to babies with brains a full standard deviation larger than would be expected for a primate of our size. What’s more, newborn weight is almost 1.5 standard deviations larger than expected, making human infants the linebackers of great ape babies. In fact, the human infant’s head closely approximates, and is sometimes larger than, the dimensions of the maternal birth canal. And once the head squeezes through, the baby’s shoulders also have to negotiate the changing diameters of the mother’s pelvis. Such a close match between baby’s size and mom’s pelvis seems like poor planning on the part of natural selection, and so the obvious question becomes, Why can’t the maternal pelvis just get bigger?
Wide hips aren’t bad for walking
For decades, the answer to this question of pelvis size was thought to lie with the function of those hip abductor muscles. When researchers tried to understand how the hip abductor muscles were working during walking and running, they used a see-saw model to explain haw gravity pulled on the body center of mass on one side of the fulcrum (the hip joint) and the hip abductors pulled, with much less mechanical advantage, on the opposite side. Under this model, it was assumed that the mechanical advantage of gravity pulling down on the body was dictated by how wide the pelvis was, and thus how far the hip joint would be from the body center of mass. If that was true, then a wider pelvis would make the hip abductors work even harder to counteract this rotational force. And if they had to produce more force, that would increase the amount of energy the muscles required, making walking and running more metabolically expensive for females.
The problem with this explanation was that the underlying biomechanical model was too simplistic and left out important forces that occur during dynamic walking and running which can alter any potential relationship between pelvic width and the rotational force exerted about the hip. Back in grad school, and pregnant again, I spent my time in the biomechanics lab collecting motion capture, force, and energetic data on men and women while they walked and ran. All the participants also had an MRI of their lower body so I could measure the size of their pelvis and other important muscle and skeletal variables. My son was born by cesarean section five days after the last subject had their MRI.
When I returned to the lab, I found the data surprising. They showed that the width of a person’s pelvis has nothing to do with how much energy their hip abductors required while they walked or ran. Furthermore, men and women in the sample (and many others in the literature) were equally efficient walking and running when controlling for body mass and distance. If pelvic width has no influence on locomotor energetics, then why are human babies required to pass through a pelvic canal that sometimes is not big enough to accommodate them?
Birth complications are…complicated
In recent years, several ideas have been put forward suggesting pelvic width is limited by other functions of the pelvis. Locomotor factors aside from efficiency, like injury or mechanical stress on the hip, have been suggested to limit the width of the pelvis. A wider pelvis might also increase stress on the muscles and ligaments of the pelvic floor that hold in the guts. But the human pelvis isn’t just a product of natural selection for particular functions. Many other evolutionary forces also contributed to the shape of the modern pelvis. Populations differ in pelvic shape due to genetic drift, the chance inheritance of particular genes that code for pelvic shape. Further complicating the story, these genes also do not act as discrete units but instead are integrated in their effects on the body. Equally, if not more importantly than these evolutionary considerations, environment has profound impacts on pelvic size and shape, and social factors shape the expectations and experience of birth. This requires us to shift our focus away from a long-ago story of competing demands on the female pelvis to a here-and-now explanation for why childbirth sometimes is marred by complications.
Under the obstetrical dilemma, the tight fit between the fetus and the birth canal is thought to be problematic during birth because it causes obstructed labor, where either the infant head or body gets stuck, preventing delivery. This complication is deadly for both mother and infant if not treated by medical professionals. Acquiring precise numbers on the frequency of obstructed labor is a challenge because definitive diagnoses during delivery are difficult, and if labor is prolonged, obstruction is often assumed. Cesarean section rates are sometimes inaccurately conflated with obstructed labor, but many medical and cultural conditions can result in delivery by cesarean. The best estimates for labor obstruction as a cause of maternal mortality place the frequency at approximately three percent globally, but rates vary considerably by geographic location and maternal injury due to obstruction can also occur if treatment is delayed. For context, hemorrhage, hypertension, and sepsis together account for more than 50 percent of maternal deaths annually, making obstruction an important but less frequent complication.
The culprits for the mismatch between the size of the birth canal and the size of the fetus are likely more cultural and social than evolutionary. One important line of research has begun to address the impact of nutrition on the development of the pelvis during growth. Vitamin D deficiency has long been known to cause deformity of the pelvis through rickets, with disastrous consequences for birth, but other nutritional deficits can inhibit growth and undoubtably have implications for adult pelvic size. Nutritional status during pregnancy is also implicated in labor obstruction as maternal obesity is associated with larger infants. In isolation or together, these nutritional impacts can set the stage for a complicated delivery, but neither is a direct result of a failure of natural selection. Rather, they are the consequence of the body’s response to its environment.
Yet there is growing evidence that poor maternal health can have multigenerational effects. Maternal childhood growth is an important predictor of birth size in the next generation, and if the mother continues to be malnourished during her pregnancy, she is even more likely to give birth to a low-weight baby. Thus, the cycle perpetuates, increasing the risk of obstructed labor. If future research confirms the link between nutritional status and obstruction and fails to directly implicate other evolutionary factors in pelvic restriction, then the obstetrical dilemma seems less like evolutionary destiny and more like a convenient scapegoat for a complicated story of interacting biological and cultural factors that shape the human birth experience.
My own complicated birth story wasn’t over. Pregnant with our third child, I went to my doctor for a checkup, and having had two emergency cesareans previously (neither for obstruction), I wanted to talk about the possibility of a natural birth. The look I got said it all. He told me how dangerous that was. He told me how he just saved the life of a patient who, against his advice, tried for a vaginal birth after a previous cesarean. I felt small and ignorant, despite all my research, and left in tears. My doctor’s reaction, and many other complicating factors unique to every woman’s place in time and space, are the problems that become hidden when we explain birth difficulty as an inevitable consequence of our evolutionary history. With a new obstetrician, I carefully weighed the risks and benefits of vaginal delivery versus repeat cesarean, and having stated she was comfortable with either, I chose to birth my daughter by C-section.
I’m so grateful to have access to the medical care that helped bring my children into the world. It wasn’t evolution that created my birth story with all its complications, but a multitude of factors unique to my place in time and space.