If you have heavy vaginal bleeding during the first month of pregnancy, you should be alerted to the possibility of miscarriage or preterm miscarriage. If it is confirmed that the gestational sac is normally in the uterine cavity, heavy vaginal bleeding at this time may be a miscarriage. If the bleeding is accompanied by severe abdominal pain, the possibility of miscarriage is very high. In the case of preterm miscarriage, vaginal bleeding may also occur, but the amount of bleeding is less than in a full miscarriage.