Why is human childbirth so difficult and risky?

Human birth is hard mainly because the fit between the baby and the mother's bony birth canal is unusually tight. The canal is also fairly rigid and convoluted, so the baby usually has to rotate and flex or extend its head on the way out. Compared with other primates, this makes birth riskier, and where midwifery and medical care are lacking, obstructed labor from a mismatch between baby and pelvis is a major cause of maternal and newborn death. Most women do give birth vaginally with adequate care, and the mismatch affects only a small share of births. [1] [2]

A review in the American Journal of Obstetrics & Gynecology traces the problem to the evolution of walking upright and of a large brain, with the birth canal shaped by trade-offs among biological, environmental and cultural factors. Medical data show that mothers with a smaller birth canal face a higher risk of obstructed labor, which favors a roomier pelvis. Against that, studies indicate that a larger canal raises the risk of pelvic floor problems such as prolapse and incontinence, which can reduce survival and reproduction. Pelvic size is only a minor risk factor for those conditions, but the link still pushes selection toward a smaller canal. [3] [1]

The baby is under opposing pressures too. A smaller fetus eases delivery, but smaller size at birth lowers infant survival. Mathematical models cited in the review suggest that a compromise between these forces inevitably involves some rate of fetopelvic disproportion. [1] [3]

The classic obstetrical dilemma holds that we need a wide pelvis to deliver big-brained babies but a narrow one to walk and run efficiently. The AJOG review notes that the idea that a wider pelvis is disadvantageous for bipedal movement has not yet been empirically verified. Some anthropologists have gone further and argued that there is no obstetrical dilemma at all, and that difficulty comes mostly from modern lifestyles and inappropriate, patriarchal obstetric practices. Part of that motivation, according to critics of the position, is to counter the pathologization and overmedicalization of childbirth. [4] [3] [1]

Defenders of an evolutionary explanation reply that failing to find biomechanical or metabolic limits on pelvic size does not erase the evidence for obstetric selection. They argue the dilemma exists but is more complex than the 20th-century version. They also note that other primates, including apes, may have a tighter fit than once assumed, but say this does not disprove human difficulty. The human combination of a very tight fit, relatively immobile pelvic joints and rotational birth appears unique among primates. The search results come largely from this defending side, so the skeptical position is described here mainly through its critics. [1] [2]

Fetal size and pelvic form are not purely genetic. Undernutrition in early life followed by overweight can stunt growth and shrink the birth canal, raising obstruction risk. Rapidly improving living conditions can speed fetal growth while maternal dimensions lag a generation behind. A New Scientist article reports researchers arguing that farming left women with smaller birth canals. [1] [5]

Assisted birth is routine in humans and very rare in other primates, and forms of it have existed since the stone age. The AJOG review suggests that by buffering selection, assistance may have slowed the evolution of a larger canal, and that cesarean deliveries have shifted the evolutionary equilibrium. [3] [1]