I have a newborn a few weeks old. They are barely "there". It's as if they need another full trimester to become a baby proper. But if women had to carry a baby an additional 3 months and then deliver it, they wouldn't survive. From an evolution perspective, they have to come at 9 months.

But in an ideal scenario, the child would have more time to develop in utero. My question is, supposing evolution supported this, what implications would this have on both the woman's and man's biology? What would the average woman look like (ie. wider hips to support a larger baby)? Would behaviors change?

This is something my wife and I talked about today and we'd love to hear informed opinions.