Metformin versus placebo in obese pregnant women without diabetes mellitus

Background: Obesity is associated with increased risk of adverse pregnancy outcomes. Lifestyle intervention studies have not improved outcome. Metformin improves insulin sensitivity and leads to less weight gain.

Methods: Our double-blind placebo-controlled trial randomized non-diabetic pregnant women with a body mass index >35 kg/m2 to metformin or placebo from 12-18 weeks’ gestation until delivery. Primary outcome was median neonatal birth weight z-score reduction by 0.3 standard deviations (equivalent to a 50% reduction in incidence of large-for-gestational-age neonates from 20% to 10%). Secondary outcomes included maternal gestational weight gain and incidence of gestational diabetes, and preeclampsia, as well as adverse neonatal outcomes. Women were randomized, by computer generated random numbers, to either daily metformin 3.0 grams (n=225) or to placebo (n=225). Analysis was by intention to treat.

Results: Fifty women withdrew consent, leaving 202 in the metformin group and 198 in the placebo group. There was no significant difference in median neonatal birth weight z-score (metformin: 0.05, IQR -0.71 to 0.92; placebo: 0.17, IQR -0.62 to 0.89; p=0.655). In the metformin group, compared to placebo, median maternal gestational weight gain was lower (4.6 kg, IQR 1.3-7.2 vs. 6.3 kg, IQR 2.9-9.2, p<0.0001) and incidence of preeclampsia was lower (3.0% vs 11.3%; odds ratio 0.24, 95% CI 0.10-0.61; p=0.001), incidence of side effects was higher; there were no significant differences in gestational diabetes, large for gestational age neonates and adverse neonatal outcomes.

Conclusions: In non-diabetic women with BMI >35 kg/m2, antenatal administration of metformin reduces maternal weight gain but not neonatal birth weight. (ClinicalTrials.gov number, NCT01273584)