Multiple births (of all orders) increased in epidemic proportions in the years 1971-1997. Twins increased 53%, 32%, 31%, and 83% in white, Afro-American, Native American and Mexican American women, respectively. Triplet, quadruplet, and quintuplet+ births increased >400%, >1,100%, and >500%, respectively, in the same years. The principal causes of these changes are related to the increasing age of the maternal cohort and an increasing incidence of fertility-inhibiting diseases and conditions in association with advancing maternal age. Major immediate consequences of these changes include disproportionately large numbers of infants born at <33 weeks' gestation (1.7% for singletons vs. 41.2% for triplets) and at