If we carried our children until they were sufficiently physically developed to stand, they'd kill the mothers in childbirth 19 times out of 20. As our brains

If we carried our children until they were sufficiently physically developed to stand, they'd kill the mothers in childbirth 19 times out of 20. As our brains became more advanced, our crania became larger and more dangerous to birth at full term. But as our intelligence increased, so did our ability to care for infants that were essentially born premature. So now even our full term babies are born so early that they need another 3 months (sometimes referred to as the "fourth trimester") just to be mature enough to sleep properly, among other things. But as intelligent humans, we're able to care for them and devise ways to carry them with us and protect and nurture them despite their extreme helplessness. FWIW, the offspring of carnivorous mammals, though not born quite as "premature" as humans, are often as underdeveloped at birth as a 3-month-old baby. I mean, have you ever seen a newborn kitten or puppy? Eyes closed, barely able to crawl to its mother's teats? The need for a prey animal - especially one that depends on speed for self-preservation - to be able to escape from within minutes of its birth is very different from the survival requirements for a carnivore or even a prey animal that gives birth in a hidden den (like many rodents).