Why do you bleed after birth?

Answer 1

After you give birth, either via cesarean birth or a vaginal birth, you will bleed from your vagina. The cause of this bleeding is the healing of the uterus, specifically where the placenta was attached to the uterine wall. As the uterus slowly shrinks back down to nearly it's prepregnancy size in a process called involution, the body expels blood form the wound inside.

Source : http://pregnancy.about.com/od/postpartumrecover/f/postpartumbleed.htm