Okay, but why wouldn't the placenta be able to feed the infant at later stages? I suspect it's because the growing head needs too much energy which must be fed by the mother at all hours of the day. What makes human babies different from other social animals is their relative brain size which is ravenous for calories. The baby's brain consumes 60% of the baby's metabolic energy.
Certainly, the womb evolved to accomodate the infant's head, but the question is whether it could have developed further to accomodate the infant at, say, 15 or 20 months when the infant doubles in weight, without disabling the mother and preventing her from walking. As it is, women sometimes die in childbirth, whereas mothers in other species have it relatively easy.
Of course, I'm no expert on any of this and could be mistaken.
