I can't remember the name (shame on me), but there's a brain chemical that's produced during childbirth that causes a woman to bond with... someone. Generally the child, or even the father of the kid.
http://en.wikipedia.org/wiki/Oxytocin
This was the one honest thing I learned in American sex education. When a woman has sex, gives birth, or breastfeeds, her body releases oxytocin that helps bond her to the sexual partner or child.