It's one of those things that happens in almost every movie. They can't use a real baby that was just born, so they take an older one and usually forget to put a lot of details.
What, were you expecting a real, new born baby to come out? Do you realize that it takes a few hours to give birth to a baby? What were they going to do, wait around all day for the baby to pop out? Plus, it would've been extremely hard for the women giving birth to act her role while giving birth! Sometimes, you just have to use your brain and accept the fact that things don't always turn out perfectly on movies. If you're watching the movie, watch it because you think you'll enjoy the MOVIE, not because you think you'll enjoy the stuff ups!