If you really want to get detailed and think about it.  Only the mating of a male vampire and a human female could ever possibly result in pregnancy.<br/><br/>This would be due to the fact that undeath halts the normal function of the human reproductive system.  For female vampires, this means they no longer ovulate nor are they fertile any more, so it wouldn't matter if they did.  For the males, they would no long produce sperm and their ejaculate in their "unllife" would essentially be their own vampiric blood.  Or at least produced from it...  So for whatever reason on the rare occasion, mating with a human female would result in a "strange" pregnancy.  The vampiric blood essential creating unlife at birth.