to be fair, the vagina does change dramatically as the birth approaches, and the real chokepoint (the cervix) stretches open over a period of a day or days (and softens for weeks beforehand). by the actual exciting part of labor, your whole crotch is pretty much already optimized to give birth, not just your uterus. that’s one of the reasons sex is uncomfortable after birth. the baby didn’t suddenly just burst everything open. your body grew that way slowly, possibly without your awareness, and takes as many days to return to normal again.
 
not that anyone should have kids. f-in monsters after they get out of you.