As humans, there are many things that set us apart and make us relatively unique from other primates. Besides our capacity for abstract thought, our mastery of fire, and our obsession with reality television, the simple fact of being bipedal has had drastic effects in our ongoing biological and social evolution. Women in particular bear the brunt of these evolutionary consequences as they go through pregnancy and birth.
The pelvis supports the upper part of the human body and distributes weight onto our legs. While this can give us mobility advantages, it also means that the female pelvis had to evolve to accommodate bipedalism, large brained neonates, and secondary altriciality. Because we are the only mammals to walk exclusively upright, we have unique birthing challenges.
In order to basically keep the fetus from "falling out" from between a woman's legs when she is standing, the pelvis developed into a unique shape. Unfortunately, this shape forces the neonate to navigate the pelvis as it is being born, basically having to complete two partial lateral rotations to accommodate first its large head, and then broad shoulders. An article entitled Birth, Obstetrics and Human Evolution by Karen Rosenberg and Wenda Trevathan says,
“The series of rotations that the human neonate most commonly undergoes during birth is related to the locomotor pattern of bipedalism as well as to...a relatively large brain.” (1)
It used to be thought that birth was relatively easy and uncomplicated for animals and that humans were unique in their difficult births. We now know that this is not the case.
“One important characteristic of primates as a group is a large head and brain relative to body size...For most primates, this means that their neonates at birth have heads that are close to the size of the maternal birth canal through which they must pass. This is especially true of monkeys, lesser apes and humans.” (2)
Traditionally it was thought that apes and monkeys had easy births because humans were the only primates whose young had to undergo rotations to navigate the birth canal. Today we know this is not always true. While not enough animal births have been observed to draw a decisive conclusion, some studies have noted neonates rotating through the birth canal, though in different ways from humans. Non-human primates still have a distinct advantage over women because their young are generally born facing toward the mother so that she can help guide them out and up to her.
What does all this mean for birthing women? Well, it’s not as bad as it sounds. Our pelvis may be perilously close to being too small and babies heads a hair's breadth from being too large, but our amazing bodies help compensate for that with the hormone Relaxin.
Relaxin helps to relax the muscles, ligaments and bones in a pregnant woman’s body, concentrating its effects on the pelvis and lower back area. While these effects cause the unpleasant “pregnancy waddle” and balance woes associated with late pregnancy, they also allow the pelvis to expand as the baby moves through the birth canal.
And if a woman's expandable pelvis wasn't amazing enough, babies are also well equipped to deal with the tight fit. Neonate skulls are soft and flexible with gaps between plates in their head. This means that the head can mold and shape itself to fit through the pelvis as needed. If it’s a tight enough fit, the bones can even overlap, causing the infamous cone head. If the baby does come out looking cone headed, its head will gradually go back to normal. So while being the only primate to walk exclusively upright has serious evolutionary drawbacks when it comes to pregnancy and birth, it also means that the human birth process is particularly amazing. Women's bodies are made to birth and babies are designed to be birthed. From an expandable pelvis to a shrinkable head, mother and child work together in an intricate, exquisite process.
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