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Abstract
Objectives: To determine the risk factors for birth weight discordance in twins. Methods: We used the United States (1995-97) Matched Multiple Birth File (n = 294 568) to assess the association between birth weight discordance and maternal sociodemographic, pregnancy and infant characteristics. Results: Eighty-four per cent of the twins were 0-19% discordant, 11.1% were 20-29% discordant, 3.4% were 30-39% discordant, and 1.8% were ≥ 40% discordant. The risk factors for birth weight discordance for same-sex twins were eclampsia (odds ratio (OR) 1.39, 95% confidence interval (CI) 1.20, 1.61), pre-eclampsia (OR 1.31, 95% CI 1.24, 1.38), pre-existing hypertension (OR 1.32, 95% CI 1.12, 1.56), diabetes (OR 1.13, 95% CI 1.04, 1.24) and certain congenital anomalies. For opposite-sex twins, the risk factors for birth weight discordance were pre-eclampsia (OR 1.17, 95% CI 1.09, 1.27), pre-existing hypertension (OR 1.59, 95% CI 1.32, 1.91), and certain congenital anomalies. Also, smoking and increased maternal age were associated with birth weight discordance in both same-sex and opposite-sex twins. Conclusions: Maternal hypertensive disorders, smoking and delayed childbearing were associated with intrapair birth weight discordance. The mechanisms of these associations deserve further investigation.