I don't know how much your daughter's question indicates she knows about childbirth; it would certainly indicate she knows that babies grow in mommy's belly. As you can well imagine, there are many different "childhood expert" suggestions for how to talk to your children about childbirth, sex, and sexuality (this last area being the one parents feel most uneasy about). I will offer an example of an age-appropriate response to your daughter's specific question and also give you some excellent web sites that focus on teaching kids about childbirth and sex. Here's the specific response: You remember when mommy/daddy told you that babies grow inside a mommy's belly in a special place called a womb. Well, at first, when babies begin to grow in the mommy's womb they are very, very small. As the baby grows bigger, the mommy's womb stretches to make a bigger home for the baby; that's why you see pregnant ladies getting bigger and bigger bellies.

When the baby has grown big enough so that it's ready to be born, she comes out through the mommy's vagina; the mommy's vagina stretches to let the baby come out and then she is born. You can use an elastic or something elastic-like to demonstrate the stretching if your daughter seems to want an additional "visual aid." Always take cues from your child as to how much information she really needs at any given time. Parents tend to either postpone these discussions altogether or give far too much information, more than the child needs or can handle.

Here are some online resources: Sex Education for Preschoolers; Teaching Your Kids About Sex; and Parent's Resource Almanac on Sexuality

Good luck with your discussions.