> Is there a reason why women should take care of newborns full time? Yes, that's what is natural (meaning occurs in nature).

A lot of things happen in nature. But is there a reason a women is required to raise an offspring? Apart from breast-milk, there isn't anything a man can't do.

> Is there any reason her husband shouldn't help her? I don't really see one.

No reason at all. I am more interested in parent's comment about woman required to take care of babies. Let alone helping, I don't see a reason why a man can't raise a baby all by himself.