Effects of fetal sex and race on risk of very preterm birth in twins

OBJECTIVE: Our purpose was to determine whether the risk of twin preterm birth correlates with the number of male fetuses. STUDY DESIGN: Among 8109 white and 1884 black twin pregnancies in the Missouri Successive Pregnancy Birth/Death Data Set, 1978 through 1990, risk for preterm birth at various gestational ages was determined with 0, 1, or 2 male infants. RESULTS: Studied as individuals, white preterm twins <35 weeks' gestation demonstrated a 9.2% excess of male fetuses (P<.001). Adjusted for monozygosity, risk for preterm birth <35 weeks' gestation was 15.7% in white female-female pairs, 17.9% in unlike-sex white fetuses, and 20.2% in white male-male pairs (r=.999, P=.01). The effect was absent in black pregnancies and was unrelated to birth order, cesarean delivery, parity, twins' weight differential, year, or season. CONCLUSIONS: In white twin gestations the observed linear relationship between the number of male fetuses and the likelihood of preterm birth <35 weeks' gestation suggests a fetal mechanism for preterm birth <35 weeks' gestation linked to fetal sex. Studies of mechanisms for preterm birth must stratify by fetal sex and race.