Human birth is a challenging process usually lasting several hours, rather than just an hour or two as in monkeys and apes. This is because our newborns are unusually large -- twice as big as babies of great apes -- and have particularly large brains.
During birth, the human baby rotates in a complex manner, with the result that its head normally ends up facing toward the mother's back rather than to the front, as in other primates. Measurement reveals that the size of the newborn's head is pushed to the limit. Biological dimensions generally fit a standard bell curve, which statisticians call a normal distribution. But the curves are truncated at the upper end for the newborn's head and at the lower end the size of the pelvic canal.
In a process called "genetic pruning," natural selection has acted against over-large heads and small pelvic canals. The fit is still so tight that the human brain also has to grow an unusual amount after birth. In other primates, the newborn's brain is about half its adult size, but in humans it is only a quarter of adult size.
So, despite the large size of human newborns and their brains, additional rapid brain growth after birth is needed.