The typical human gestation times of 38 weeks is, in fact, longer than one would expect for a primate of similar body mass. Instead of arguing for an enlarged brain, then, one might as easily argue for a diminished body mass. Thus, it is in spite of similar periods of gestation that humans are more helpless than other great apes at birth; humans are not born early. Moreover, we wean infants earlier than expected for a primate of our body size, not later. The opposite should be the case if weaning time is a proxy for altriciality, and if the obstetrical hypothesis is correct that altriciality requires more intelligent parents.
Fetal development provides additional evidence for the metabolic hypothesis. Human infants have brains roughly one third the size of adult brains; for chimpanzees, the figure is much higher, at roughly 40%. At 16 weeks of gestation, the human brain is twice the size of the chimpanzee brain. After 22 weeks of gestation, chimpanzee rates of growth slow and human rates of growth accelerate. At 400cc, human newborn brains are roughly 2.7 times larger than chimpanzee brains. Since maternal energy demands increase exponentially during human fetal development, waiting longer than nine months would exceed the maximum sustainable maternal metabolic rate, which is approximately the same across primates. Other mammals rely on progesterone depletion for the initiation of birth, but primates give birth when progesterone levels are at their height. It is an energetic ceiling that signals the onset of labor.More.
A good question. Different mammals are born helpless in different ways. Kittens are born more helpless than humans in some ways because their eyes and ears are sealed shut or squished for a couple of days/weeks, during which they stumble around, finding a teat only by smell. Yes, they can move, but communication via eye contact is very limited. On the other hand, the iconic feline eyes and ears may need protection from the rigors of a birth canal.
See also: Haldane’s dilemma is still really a dilemma (Chase Nelson)