"Who's The Daddy?"
Wow. Did you really not know this question was off-limits? If you'd like to avoid the half-eaten enchilada (or whatever snack happens to be at hand) surely being aimed at your head, steer clear of this lineage-based query. The truth is, if you're unsure who this baby's father is, you probably don't know her well enough to ask.
Of course, if a pregnant woman is in your circle of family and friends, you know exactly who the father is, and are simply making a joke. If she's a veritable stranger, we won't feel a bit sorry for you when that enchilada hits. The same goes for referring to the baby's father as the "baby daddy." Seriously?