A Question from Three Years and Five Months Toddler

Dave sees lots of pregnant women these days. These women always tell him about the baby in the stomach. Some of the women with early pregnancies, the tummy are not that big yet, and some are really showing already. He is aware that there is baby in the tummy. So, I were pretending to tell him that there is baby in mommy's tummy. Then he will really show his love and talk to the "baby" in the tummy. This morning, he was kinda pointing to my breast and asked "So, who is in there?"......... ('coz he thinks that everything big has baby inside... ^^)
Honestly, I were quite stunned and I didn't know what to answer. So, after a few seconds, finally I told him that there is nothing in there. The babies are exists only in the tummy.

When is the best time to educate him about human anatomy?