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paleoanthropology, genetics and evolution

Photo Credit: Newborn human baby, by Ernest F. (via Wikimedia) CC-BY-SA 3.0

Dilemma of the obstetrical dilemma

During the past few years, anthropologists have been questioning the long-held idea that human birth is uniquely risky for mothers and infants because of the narrow size of the human pelvis. This week, Josie Glausiusz has an article for Undark that reviews the topic: “Of Evolution, Culture, and the Obstetrical Dilemma”.

The assumption that “women are compromised bipedally in order to give birth,” is widely accepted says anthropologist Holly Dunsworth of the University of Rhode Island. But Dunsworth sees flaws in this premise. Women already have a range of dimensions in their birth canal, she thought, and they are all walking just fine. Indeed, research on human skeletons by anthropologist Helen Kurki of the University of Victoria in Canada has shown that the size and shape of the human birth canal varies very widely, even more so than the size and shape of their arms.

The article provides a fair summary of the conversation about birth and human evolution happening now in the field. It focuses on Dunsworth’s ideas about metabolic limits on gestation, and Jonathan Wells’ hypothesis that most complications with birth seen today are results of postagricultural changes to human nutrition and subsistence patterns.

The topic of the obstetrical dilemma illuminates many ways that people become confused about what evolution means to us in our lives.

Human babies are born relatively helpless compared to other primate babies, and they are born with a smaller proportion of their adult brain mass, leaving more of their brain growth for the first year of life. But human babies are not born early compared to other primates. Adult body mass predicts gestation length in primates pretty well and human babies are born about when we would expect for a primate with human body mass.

People make a huge deal out of the difficulty of human childbirth. Traditional people around the world recognize that human childbirth is difficult compared to many other animals. In the Christian tradition, the difficulty of human childbirth is even recognized in the bible, with Genesis 3:16 saying, “I will greatly multiply Your pain in childbirth, In pain you will bring forth children.”

Still, although childbirth can be very difficult for both mother and child, the extent of this difficulty varies greatly among women and among births by the same mother. Pain is hard to compare across species, as non-human primates cannot report what they are experiencing during uterine contractions.

The dimensions of the pelvic inlet and the average newborn head are much easier to compare objectively. Compared to great apes like chimpanzees and gorillas, human infants have larger heads, and the maternal pelvic inlet is much smaller. But many other species of smaller primates also have relatively large infant heads compared to the maternal pelvic inlet:

Infant head size at birth compared to maternal pelvis size, from Rosenberg and Trevathan 1995
Infant head dimensions at birth (black outline) compared to average maternal pelvic inlet dimensions (open outline) in various primate species and humans. This figure is from Rosenberg and Trevathan (1995), using data from Adolph Schultz.

What makes humans different from macaques, as Rosenberg and Trevathan pointed out, is not only the small size of the maternal pelvic inlet relative to infant head size, but also that its long axis is side-to-side instead of front-to-back. This means that most infants must rotate as they pass through the birth canal, while the smaller primates are typically born with the back of infant heads facing toward the back of the mother.

Still, the large heads of small primates show that there’s nothing inherently paradoxical about the human “obstetrical dilemma”. Trade-offs shape the timing of life history events. In both small-scale human societies and in primates, mortality at the time of birth is slight compared to infant mortality during the first year of life. Babies could be born earlier and smaller, but both have substantial costs that balance the occasional mortality from cephalopelvic disproportion.

References

Rosenberg, K., & Trevathan, W. (1995). Bipedalism and human birth: The obstetrical dilemma revisited. Evolutionary Anthropology: Issues, News, and Reviews, 4(5), 161-168.