Prolonged gestation is not generally associated with problems or extra large foals. If your mare's pregnancy extends much past 340 days or you're concerned, ask your veterinarian to examine her to determine if the mare is still pregnant and confirm that all is well. If you notice a vaginal discharge or dripping milk during pregnancy, contact your veterinarian. If you find the remains of a placenta or fetus, save it for your veterinarian to examine. It may be possible to ascertain the cause of abortion and treat the mare accordingly.
Mares can and do abort without ill effects.
However, it's always a good idea to have her checked by your veterinarian, because some complications of abortion, such as a retained placenta, can be life-threatening to your horse. There are obvious as well as subtle signs of impending birth. The time frame during which they occur varies from mare to mare. To prepare, brush up on your foaling knowledge with the companion AAEP educational brochure, Foaling Mare and Newborn. Your veterinarian will be happy to supply it and will also be able to answer any further questions you may have about caring for your expectant mare.