I think that there are a couple of ways that they can abort pregnancies depending on how far gone they are.
If they havent been pregnant for long I believe that they can re-absorb the babies which means there will be no gravid spot visible as this dark area is essentially the eyes of the fry forming.
Alternatively she could expel the part-formed fry in which case there would again be no gravid spot and very likely no fat tummy either.
Do you think that one of your fish is aborting? What symptoms are they showing?