Miscarriage, the loss of a pregnancy generally in the first 12 weeks, occurs in at least 15% of all pregnancies. It occurs much more frequently in the pregnancies of women aged 37 or older. With very few exceptions, miscarriage occurs because the developing embryo is abnormal in some way. Chromosome abnormalities cause about 50% of miscarriages but those with normal chromosomes have the same range of developmental problems. But the chromosome problems in the embryo that miscarries are almost always caused by the mother or father's health at the time of conception.