Miscarriage, also called spontaneous abortion, is the accidental or natural ending of a pregnancy before a baby can live outside its mother's body. Studies indicate that 15 to 20 percent of all diagnosed pregnancies end in miscarriage. But the risk of miscarriage is highest during the first two weeks following conception (fertilization), a time at which most women do not even know they are pregnant. Thus, many physicians believe the overall rate of miscarriage, including undiagnosed pregnancies, may actually be as high as 50 percent.