A good friend, who has a DS the same age as mine has had eight miscarriages since I got pregnant with DD (we were originally pregnant at the same time again). Perhaps you'd like to ask her your staggeringly sensitive question?
FWIW, the best answer the specialists can come up with in her is "probably some immune factor that we don't know enough about to be able to pick up on the screening tests yet; if we give you a massive dose of steroids every month then it MIGHT help"