Use of assisted reproductive technology is a major risk factor for multiple births. The incidence of dizygotic twinning increases with a family history of twins, maternal age (peak at 35–39 years), previous twin gestation, increasing parity, maternal height, fecundity, social class, frequency of coitus, and exposure to exogenous gonadotropins, clomiphene, or in vitro fertilization. The risk of twinning decreases with undernourishment. Ethnic background (African Americans > Caucasians > Asians) is a preconception risk factor for naturally conceived multiple gestation births.