General Discussion

And I remember having my Barbie walking around knocked up all the time. We had a local girl who heard way too many inappropriate things at home and would promptly share them so Barbie was "knocked up" with several other children and we'd pretend she was smoking, drinking and yelling at the kids to go outside. She once told us about how her older sister who was pregnant "peed herself" and started screaming about the baby coming. We in turn poured water on Barbie and did the same.
It was the Seventies.

In other words, kids are strange and fascinated with the oddest things. The bigger the reaction the more likely they'll want to do it. In this case look around for a good storybook about having a baby, read it and be open for questions at any time.