Body mass index change between pregnancies and small for gestational age births.

OBJECTIVE: To estimate whether maternal weight changes between pregnancies influence the risk for small for gestational age (SGA) births. METHODS: SGA cases (n = 8,062) below the tenth percentile birth weight for gestational age were selected from liveborn singletons born of Missouri residents during 1989-1997. Normal weight controls (n = 8,062) were selected according to birth year. The risk of SGA from interpregnancy body mass index (BMI) change and other maternal factors was estimated using logistic regression analysis. RESULTS: An increase in BMI between pregnancies decreased SGA risk (adjusted odds ratio = 0.8; 95% confidence interval 0.7, 1.0). Other risk factors were prior SGA (4.4; 4.0, 4.8), preeclampsia/eclampsia (2.6; 2.1, 3.2), maternal cardiac disease (1.8; 1.1, 2.9), inadequate weight gain (1.9; 1.8, 2.2), and cigarette smoking (1.9; 1.7, 2.3 for 1-9 cigarettes per day; 2.5; 2.2, 2.8 for 10-19/d; and 2.8; 2.5, 3.3 for 20/d or more). CONCLUSION: Increase in interpregnancy BMI lowers SGA risk, but adequate weight gain during pregnancy is more effective.