Birds and the bees...sort of

Samantha and I were driving home from the hospital today when she asked me if Izzie's (our neighbor) Mommy was going to have a baby sister in her belly. I told her not right now she isn't. She wanted to know how come. I mumbled something about I don't know. Well then Sami asked me how I got a baby sister in my belly. Oh no, here we go! I told her that Mommies have lots of little tiny eggs in their belly and they can sometimes turn into babies. That was the end of the conversation so I guess that was a good enough answer. How the heck do you tell a (almost) 4 year old how a baby gets in your belly?