A loss of a pregnancy before the 20th week of pregnancy. But most happen before the pregnancy is 12 weeks along. Miscarriage occurs commonly. About 15% to 20% of known pregnancies end in miscarriage. The most common reason is failure of a fertilized egg to progress to an embryo or genetic changes that occur early on in the fetus. A common sign of miscarriage is vaginal bleeding, but many pregnancies with bleeding do not end in miscarriage.