K.B.
My son is 3 and we were talking about the baby coming out when is has finished growing and he asked me where the door was - LOL. So, I sort of used that as a starting point. I just told him that there is a special door at the bottom of mommy's belly that can only be opened when the baby is finished growing and only the doctor knows how to open it and let the baby out with special care. He was satisfied with that.
On the topic of how the baby got there, if and when he asks, I thought I would tell him that mommy and daddy love each other so much and that sometimes their love comes together and when taken care of, grows into a little baby so that they can share this love with the world. However, it has to be kept in a special place and taken care of until it grows big enough to be able to come out on its own. So, mommy takes care of it until it's ready to meet the rest of the world. I felt like it was the truth, but also on his level and something that I wouldn't worry about him sharing.