Meyer abordou especificamente esse ponto em uma FAQ em seu site oficial. Desculpas pela duração da cotação, mas é tudo moderadamente relevante.
Em resumo, vampiras do sexo feminino não podem engravidar. Vampiros machos são capazes de sustentar uma ereção e engravidar mulheres humanas devido à presença de " veneno " inundando suas células.
Vampires and pregnancy: when did that idea occur to you? How does that work?
I was always very careful when I answered the “Can vampires have
babies?” question, because I didn’t want to say anything incorrect,
but I also didn’t want to make the future super-obvious. I focused my
answers on the female half of the equation—female vampires cannot have
children because their bodies no longer change in any aspect. There is
no changing cycle to begin with, and their bodies couldn’t expand to
fit a growing child, either. I purposely evaded answering the
question, “Can a male vampire get a human female pregnant?” to
preserve a tiny bit of surprise in the last book. There were many
statements on this subject purported to have come from me, but I never
made those comments because, obviously, I knew where this was going.
Vampires are physically similar enough to their human origins to pass
as humans under some circumstances (like cloudy days) ... A fluid similar
to the venom in their mouths works as a lubricant between the cells,
which makes movement possible (note: this fluid is very flammable). ... The normal reactions of arousal are still present in
vampires, made possible by venom-related fluids that cause tissues to
react similarly as they do to an influx of blood. Like with vampire
skin—which looks similar to human skin and has the same basic
function—fluids closely related to seminal fluids still exist in male
vampires, which carry genetic information and are capable of bonding
with a human ovum.
Frequently Asked Questions: Breaking Dawn - www.stepheniemeyer.com