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.