Have you ever wondered why women stay pregnant for nine months? For decades, anthropologists have explained the timing of human gestation and birth as a balance between two constraints: the size of a women’s hips and the size of a newborn’s brain. But new research says that’s not the case. Instead, the timing of childbirth occurs when women’s bodies can no longer keep up with the energy demands of pregnancy. That happens at around nine months, Holly Dunsworth of the University of Rhode Island and colleagues report online August 27 in the Proceedings of the National Academy of Sciences.
The traditional explanation of gestation length is known as the obstetric dilemma. The hypothesis suggests that the width of the pelvis, and thus the width of the birth canal, is limited by the demands of efficient upright walking. But as brain size expanded over hominid evolution, heads got bigger. To make sure a baby’s head could fit through the birth canal, gestation decreased and babies were born at an earlier stage of development; today, newborns enter the world with the least developed brain of all primates at less than 30 percent adult size.
Dunsworth and her colleagues wanted to see if they could find any actual evidence to support the obstetric dilemma. First, they considered gestation length. Traditionally, human gestation has been considered short when looking at how much additional growth the brain needs to reach adult size. But such a measure is unfair when compared to other primates since humans have abnormally large brains, the researchers say. Instead, Dunsworth’s team compared gestation length to maternal body size and found humans actually have relatively long pregnancies—37 days longer than would be expected for a typical primate our size. Our gestation is also relatively extended compared with chimpanzees or gorillas, suggesting pregnancies got longer, not shorter, in hominids.
The team also looked for evidence that widening the pelvis to accommodate bigger brained babies would make walking less efficient. Researchers have assumed that broadening the hips would increase the force needed by hip muscles to walk and run, thus making locomotion less energy efficient. But one recent study shows the dimensions of the hips don’t actually affect the muscle’s required force, calling into question the long-held belief that wider hips would interfere with women’s walking. Furthermore, the team calculated how much wider the hips would have to be if humans were born with the same brain development as chimps (40 percent adult size). All that would be needed is a three-centimeter increase. Women’s hips already vary by three or more centimeters, the researchers say, suggesting that hip size really doesn’t limit gestation.
Instead, gestation is determined by energy. Studies of mammals show that during pregnancy females reach their species’ “metabolic ceiling,” the upper limit of the amount of energy they can expend. In humans, the metabolic ceiling is 2 to 2.5 times the baseline amount of energy needed during rest. Dunsworth and her colleagues say women reach that limit by their sixth month of pregnancy. Then at nine months, the energy demands of a fetus go beyond this metabolic threshold. “Extending gestation even by a month would likely require metabolic investment beyond the mother’s capacity,” the team writes.
But even though hip size doesn’t appear to limit the size of a baby’s head, women around the world often have trouble delivering babies because of the tight fit of the head going through the birth canal. One possible explanation is that childbirth has only become problematic recently in human evolution. Changes in diet that have led to increased energy consumption may be allowing women to produce bigger babies, and natural selection hasn’t had enough time to broaden the hips. Figuring out why modern childbirth is so difficult, and dangerous, is an area that needs further research.