Compared to any other primate, Human childbirth can be considered to be the most challenging. But why is that? Why does a mother face so many challenges even now after millions of years of evolution and adaptation? Is it necessary after all?

Sherwood Washburn proposed a widely accepted explanation for the tight squeeze baby experiences passing through the birth canal.

In man adaptation to bipedal locomotion decreased the size of the bony birth-canal at the same time that the exigencies of tool use selected for larger brains. This obstetrical dilemma was solved by delivery of the fetus at a much earlier stage of development.

Washburn thus meant childbirth has to have a tradeoff between adaptations for upright walking and development of the large brain. Birth at a much earlier stage holds true only for the brain. In the case of great apes (orangutans, gorillas, chimpanzees) the newborn’s brain is already about half as big as in adults, whereas in humans it is barely more than a quarter.

An anatomist Vyvyan Howard also stated in 1981:

By being born earlier, the fetal head…can pass through the maternal pelvis and then continue developing in the postnatal period.

However — despite  similar pregnancy lengths — human newborns are almost twice as big as those of great apes, so overall fetal growth is actually greater in humans and the brain is larger at birth.

THE TROUBLE STARTS WITH PASSAGE THROUGH THE PELVIS!

The birth canal in a woman’s pelvis is convoluted. Its inlet is widest from side to side while its outlet is largest from back to front. Karen Rosenberg showed in 1992 that safe passage of the newborn through the pelvis requires a 2-phase turning sequence. As the infant’s head enters the inlet, it is already rotated so that its long axis is oriented side-to-side, not back-to-front as is typical in other primates. Passing on through the pelvis, the baby’s head then rotates further to fit the longer front-to-back axis of the outlet. So the newborn typically faces toward the mother’s back on emergence. Other primates usually lack rotation and the baby passes through the pelvis with its head facing forward. In fact, it is not only the human newborn’s large head that makes birth challenging. Its shoulders are also quite wide compared to the birth canal, so additional juggling is needed to avoid jamming.

Birth canal image from Internet Archive Book Images [no restrictions] and Henry Vandyke Carter [Public domain], both via Wikimedia Commons.

Infant’s head entering birth canal (left) and associated male/female differences in pelvis shape between (center and right). In women. the pelvic shape is adapted to maximize the size of the canal and minimize obstructions

WHY HASN’T EVOLUTION HELPED IN EASING THIS PROCESS?

Available evidence indicates that development within the mother’s womb is favored as far as possible over the development of the suckled infant after birth. Perhaps it is simply more efficient to transfer nutrients directly across the placenta, instead of first converting them to milk that the infant must digest. Selection may also maximize time in the womb because it provides protection and environmental stability. Whatever the reason, human birth clearly occurs at a point where the baby will only just pass through the pelvic canal.

Source: Psychology Today

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