L.S.
When you give birth your uterus is all stretched out and you are swollen. (Think about all of the extra water and hormones you are carrying around) This is not a good time to have plastic surgery.
Also you will have an incision on your uterus- so if you end up having problems from the tuck it could possibly affect your uterus too.
It sounds like a good idea in theory, in fact I would have signed up for a boob job and a hysterectomy with my last child- but waiting is better.