The incidence of congenital heart disease appears to be about 1 per 100 liveborn infants. In infants who die before term, however, there is a much higher incidence of congenital heart disease, with a tendency for an excess of complex lesions. Some but not all of these lesions are associated with gross chromosomal abnormalities, which occur frequently in first-trimester abortions. Most of these chromosomal abnormalities are associated with such maldevelopment of many organ systems that fetal death occurs in utero. Monosomy X (45, XO), has a high association with congenital heart disease. Most fetuses with this abnormality die in utero, but because the abnormality is not inevitably lethal a small increase in survival of these fetuses would cause a large increase in the total incidence of congenital heart disease.