Maternal factors in dizygotic twinning: evidence from interracial crosses

In the USA, Blacks have been shown to have a higher incidence of DZ twinning than Whites. We studied maternal and paternal race determinants of the rates of like-sex and unlike-sex twinning using US live-birth certificate data for 1973-78. After adjustment was made for father's race, Black mothers had a higher rate of unlike-sex twinning than White mothers (odds ratio, OR = 1.74, P less than 10(-8]. This maternal race effect persisted after further adjustments were made for maternal age, parity, education, and marital status and did not apply to the rates of like-sex twinning (OR = 1.06). On the other hand, after adjustment was made for mother's race, Black fathers did not have higher rates of unlike-sex (OR = 0.93) or like-sex (OR = 1.11) twinning than White fathers. The study shows that the high rate of DZ twinning in Blacks is associated with maternal race and not paternal race. The data further support the importance of maternal determinants of dizygotic twinning in humans.