What accounts for a human infant's head being about as big as it can be and still pass through the birth canal?

Question

Quizplus · Accepted Answer

The upright posture of humans has led to a narrowing of the birth canal compared to the pelvic opening of our primate ancestors. As a result, human infants must have a head size small enough to fit through this narrow canal, which is why their heads are about as big as they can be and still pass through during birth.

What Accounts for a Human Infant's Head Being About as Big