When my son was in preschool, I went to pick him up one day and the teacher took me aside and said, "we were reading a story about a mom and how she had a baby in her tummy, and your son raised his hand and said, 'That's not true. It's in her uterus!'"

I've never had "the talk" with him, I talked to him continuously through his childhood. As he asked questions, I gave him answers.