Limud Torah

The Mitzva of having children is to have a boy and a girl. If someone gives birth to a Tumtum (who's organ is covered by a membrane and we can't tell if it's a boy or a girl) he's not Yoitzeh the Mitzvah (with another child,) even according to the Rambam that holds that from the Torah, even if one is in doubt whether he had fulfilled a Mitzvah or not, he's Yoitze (but the Rabanan requires him to make sure the Mitzvah is definitely done.) Since the child as is cannot reproduce, so it doesn't count towards the Mitzvah of P'ru Ur'vu.