What should I do or say?

I was watching the movie "Milo and Otis" with my 5yo niece the other day (it's about a real cat and dog on an adventure). Nearing the end of the movie, it showed the cat and the dog giving birth. The scene was only 20 seconds or so, but it showed it all. After seeing that, she asked "how/why are they pooping the babies out?" She knows that babies come from "mommy's tummy" but that's it. She doesn't know how they are "born" (via vagina), nor does she know how babies are made or about sex (thank god, I'll keep her innocent as long as possible). She watched the movie again later and she kept rewinding back to the birth scenes and again she asked me. I don't want to ignore her question and I don't want her to think that I don't know (I'd rather she learn it properly from me than anyone else), but what do I say?