Oops, sorry, what I meant to say was that I don't think there's any medical theory out there that would account for the connection between pregnancy and nightmares, or a physiological state and the content of dreams generally. The most that psychology seems to offer is that we pregnant ladies have a lot of serious changes going on and reasonable(as well as unreasonable) fears about how things will go, and so this increased anxiety gets expressed in negative dreams. Hopefully the wonderful aspects of pregnancy are stimulating other, happier dreams as well, which don't get fixed in our memory because it's only the nightmares that wake us up enough to do any remembering.
I do find myself a lot more philosophical about these recurring pregnancy dreams gone awry than I do about the full-on, sweat-inducing, heart-palpitating nightmares that occasionally happen when I'm not pregnant. That's why I thought maybe there was some physiological difference between the two, but I guess maybe not.