The Impact of Interpregnancy Weight Change on Birthweight in Obese Women

Abstract

More than 25% of US women are obese when they become pregnant, which contributes to several known adverse outcomes. This population-based, cohort study was undertaken to determine the impact of interpregnancy body mass index (BMI) change in obese pregnant women on the risk of delivering a large- or small-for-gestational age (LGA, SGA) infant in the next pregnancy. Data from the Missouri maternally linked birth and fetal death registry were used. During the study years (1998–2005), 10,672 women started their first pregnancy with a BMI of 30 kg/m2 or greater. The final cohort included 10,444 women whose data were complete. The study groups included women who decreased their BMI by 2 or more units (weight loss group), women who increased their BMI by 2 or more units (weight gain group), and women who maintained their BMI within a 2-unit loss or gain (reference group). The prepregnancy BMI of the first pregnancy was categorized as class I (30–34.9 kg/m2), class II (35–39.9 kg/m2), or class III (≥40 kg/m2). Gestational weight gain (GWG; in pounds) for the second pregnancy was determined. Demographic, medical, and obstetric characteristics in the second pregnancy were compared across interpregnancy BMI change groups with the χ2 test for categorical variables. The incidence of LGA by interpregnancy BMI change was assessed by linear-by-linear χ2 test for trend. P < 0.05 denoted statistical significance. The final cohort included 1739 women (16.6%) who lost 2 or more BMI units between pregnancies, 4743 women (45.4%) who maintained their BMI (gain or loss, <2 BMI units), and 3962 women (38.9%) who gained 2 or more BMI units between pregnancies. Compared with the reference group, the weight gain group had higher GWG in their first pregnancy (median of 32 lb [interquartile range {IQR}, 22–42 lb] vs reference group 25 lb [IQR, 16–34 lb]; P < 0.001) and a longer interpregnancy interval (23.8 months [IQR, 14–37 months] vs 19.3 months [IQR, 12–29 months]; P < 0.001). Women in the weight loss group had a higher BMI at the start of their first pregnancy (34.5 kg/m2 [IQR, 30.2–38.8 kg/m2] vs reference group 33.7 kg/m2 [IQR, 31.6–37.3 kg/m2]; P < 0.001) but lost weight between pregnancies, resulting in a significantly lower BMI at the start of their second pregnancy (29.8 kg/m2 [IQR, 27.4–33.7 kg/m2] vs reference group 34.0 kg/m2 [IQR, 31.8–37.6 kg/m2] and weight gain group 39.0 kg/m2 [IQR, 36.1– 42.8 kg/m2]; P < 0.001). Compared with the reference group, diabetes mellitus and preeclampsia were more and less prevalent in the weight gain and weight loss groups, respectively. Interpregnancy weight change was not associated with alterations in gestational age at delivery, risk for cardiac disease or chronic hypertension, history of LGA or SGA infant, or adequacy of prenatal care. The overall incidence of LGA and SGA infants in the second pregnancy was 18.1% and 5.2%, respectively. Infant birth weight increased with parity, but despite this increase, second pregnancy offspring of women in the weight loss group were on average 56 g lighter compared with the reference group (median, 3459 g [IQR, 3124–3771 g] vs 3515 g [IQR, 3175–3827 g]). Infant birth weight was not affected by interpregnancy weight gain. In dose-response analysis, the incidence of LGA increased linearly from 13.7% in women who lost 8 or more BMI units to 19.1% in women who gained 8 or more BMI units (P value for χ2 linear trend <0.001). A similar dose-response relationship was not demonstrated for SGA infants (P = 0.84). The odds of delivering an LGA infant increased linearly with interpregnancy BMI change. The odds of delivering an SGA infant increased only in women who lost 8 or more BMI units (adjusted OR, 2.18; 95% CI, 1.32–3.60; P = 0.002). Relatively small interpregnancy changes in BMI can significantly alter the risk of delivering an LGA infant in obese women. A weight loss of 2 to 4 BMI units can significantly decrease the risk of an LGA infant in the subsequent pregnancy without increasing the risk of SGA delivery. The interpregnancy interval may be a crucial period for targeting weight loss in obese women.