Evolution

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This picture depicts the birth process of the modern chimpanzee (Pan), Australopithecus (A.L. 288-1, aka Lucy), and the modern human (Homo). As you can see from the illustration, chimpanzee infants can fit easily through the birth canal, and when the baby emerges, it is facing the mother who can pull the baby out by herself and guide the baby to her breast. Notice the Australopithecine fetal head had to rotate to fit through the birth canal and the baby emerged sideways, possibly requiring assistance during delivery.

There are three distinguishing characteristics that separate modern human births from our nonhuman primate cousins: a narrow maternal pelvis as a requirement for bipedalism, a large fetal brain, and secondary altriciality. Due to these attributes, the human birth process requires the baby to rotate while in the birth canal, allowing for the passage of the infant’s skull and shoulders. When the baby emerges, it is in an occiput posterior position, requiring assistance from another person to help guide the baby out and unwrap the umbilical cord if necessary. Due to these anatomical differences, human births are a social activity, while nonhuman primates seek solitude.

It has been suggested that human babies need to be born before they are fully developed since the infant's large brain and the mother's narrow birth canal are such a tight fit, however an alternate theory has recently developed using the constraints of metabolism as the cause. Human babies are extremely underdeveloped at birth compared to other primates. Exterogestation, sometimes referred to as the fourth trimester, describes the first three months of a human infant’s life, which resembles the rapid growth, and extreme helplessness of a fetus.

To read more about human birth from an evolutionary anthropological perspective:

http://onlinelibrary.wiley.com/store/10.1002/evan.1360040506/asset/1360040506_ftp.pdf?v=1&t=hbq1gzcr&s=f841b948566f3e2fa99647a74db903594a581779

http://www.uri.edu/news/releases/?id=6358