Jesse, welcome to the forums. I'm so sorry you've had to find us. This can be a brutal disease, and nothing can really be done about it but deliver, regardless of gestational age. It's a primary driver of maternal and neonatal mortality worldwide, as well as a primary cause of prematurity.
It might help to make some sense of this by thinking of it as about the most complex thing human bodies do. The placentas the two of you created together were formed from the unique combination of the maternal uterine lining and the paternal trophoblast (the part of the initial blastocyst that doesn't turn into embryo, where the maternal genes have been silenced.) Sometimes that process doesn't progress correctly. Sometimes twins are just too much for her body to support. Either of those things can make the foreign placenta visible to her immune system, and since foreign organs trigger immune rejection, things get worse very, very rapidly.
What underlying condition do the two of you have that necessitates LMWH injections in a first pregnancy? We have posters here with APS and lupus and so forth, who could weigh in with their experiences.
The area where a placenta has previously implanted is easier for subsequent placentas to implant into, because the uterine spiral arteries have been remodelled by the first placenta to provide blood more readily. This means subsequent pregnancies -- especially of singletons -- are very likely to go better. While anyone with a history of preeclampsia is at more risk to develop it again, it is often much later in pregnancy, and much more mild. (Some women with underlying immune responses to the paternal antigens might see earlier onset, however.) The paternal genes governing the trophoblast will also be different in any pregnancy, because they amount to a random half of your genes. The maternal response to your genes will *also* be different, because one thing every trophoblast does is modulate the maternal immune response, in permanent fashion.
Please holler with any questions; this place is huge, but we can help you find out what you need to know, and provide what support we can give. There are likely to be posters near to you, as well.