Twin birth weight discordance and risk of preterm birth

OBJECTIVES: Our purpose was to determine whether birth weight discordance is a risk factor for preterm birth of twins, and to further characterize the relationships involved. STUDY DESIGN: Maternally linked 1978-1990 Missouri birth certificates were used to analyze gestations resulting in live twins. We used contingency tables and multiple logistic regression. RESULTS: The degree of discordance correlated strongly with risk for live preterm birth but only for discordances >30% and preterm birth at <32 weeks' gestation. Among 9479 pregnancies with discordance <30%, 9.5% ended in birth at <32 weeks' gestation, versus 13.7% of 326 with discordance of 30% to 40% (P = .03) and versus 34.1% of 126 with discordance greater than or equal to 40% (P < .001). There were 42 preterm twin births at <32 weeks' gestation with discordances greater than or equal to 40%. Of these, 51% were attributable to fetal growth restriction and 16% to large size for gestational age in one infant, in 72% the smaller twin was the second born, and in 86% the twins were like sex, The relative association between greater than or equal to 40% discordance and preterm birth at <32 weeks' gestation was strengthened (final odds ratio, 9.54; P < .0001) in a multivariate model containing other risk factors for delivery at <32 weeks' gestation: black race, either twin small for gestational age, unmarried, teenage mother, number of male fetuses, like fetal sex, education <12 years, nulliparity, and cigarette smoking. CONCLUSIONS: Twin birth weight discordance has now clearly been demonstrated to be a risk factor for preterm birth. The effect was found particularly with discordances greater than or equal to 40% before 32 weeks' gestation. Discordance was usually attributable to fetal growth restriction, most often in the second-born twin.