What should be expected from the interaction between vampirism and pregnancy depends on what characteristics one chooses for the vampiric infection, and may also be affected by one's suppositions as to whether a fetus should be considered human and/or alive. As an entity with a firm attachment to its hide in an intact condition, I hasten to add that these musings represent the purest speculation. No true wanderers of the night have any cause for outrage, regardless of their position in the choice/life/whatever street riot. The strains of vampirism tend to run along a continuum from biology to possession. The more mystical variants should be just as likely as other forms of magic to act on individual selves and lives, whose definition may become important.
Some types of vampirism, of which the infections described by P.N. Elrod or Lee Killough may be considered typical, operate very much like conventional diseases. Among humans, some pathogens cross the placenta and affect the fetus while others do not. The outcome of the vampiric conversion of a pregnant woman would probably depend on whether or not the entity responsible for the infection does so. If it does, the fetus should be sustained by the infection quite as well as the woman would be. The fetus might or might not ever be born, but there is no reason to expect a miscarriage. If pregnancy progresses, the infant should be a vampire also. If the infection does not cross the placenta, a female vampire, almost certainly non-breathing, would be unable to support a still-human fetus. It would die, and, one _hopes_, miscarry promptly.
Ricean vampires are infected with the spirit of a poltergeist that invaded the ancient queen Akasha. Many other wanderers of the night share a similar condition, in which the infection is not a living organism but a ghost or force transmitted according to more or less arbitrary rules. Here, one also ought to enquire whether the pregnant woman is (1) one life carrying another equivalent life, (2) a single entity, in which one life encompasses all, or (3) one life carrying a non-equivalent life or potential life in a relationship akin to parasitism.
Spirit model (two equivalent lives):
The woman, not the fetus, is being infected. The infection could behave like either the placenta-crossing or non-crossing biological infections above.
Spirit model (one life):
If the pregnant woman is a single life, a symbiotic entity, the vampiric spirit should maintain the unit in an unchanged condition... permanent pregnancy. Ouch.
Spirit model (two nonequivalent lives):
This might result in miscarriage. If it did not, the outcome would imply that *all* of the living occupants of the human body participate in the vampiric transformation. The normal bacterial of the human intestine, any possible mundane pathogens, and any possible parasites, would all be transformed. Vampire tapeworms, anyon