In the MiTy (Metformin in Women With Type 2 Diabetes in Pregnancy) randomized trial of metformin versus placebo added to insulin, we found numerous benefits with metformin but identified an increased proportion of infants who were small for gestational age (SGA). We aimed to determine the predictors of SGA in order to individualize care.


Using logistic regression, we assessed baseline maternal characteristics as predictors of SGA. We compared maternal/neonatal outcomes in SGA metformin and placebo groups using the t, χ2, or Fisher exact test, as appropriate.


Among the 502 mothers, 460 infants were eligible for this study. There were 30 infants with SGA in the metformin group (12.9%) and 15 in the placebo group (6.6%) (P = 0.026). Among SGA infants, those in the metformin group were delivered significantly later than those in the placebo group (37.2 vs. 35.3 weeks; P = 0.038). In adjusted analyses, presence of a comorbidity (chronic hypertension and/or nephropathy) (odds ratio [OR] 3.05; 95% CI 1.58–5.81) and metformin use (OR 2.26; 95% CI 1.19–4.74) were predictive of SGA. The absolute risk of SGA was much higher in women receiving metformin with comorbidity compared with women receiving metformin without comorbidity (25.0% vs. 9.8%).


In this study, we observed a high percentage of SGA births among women with type 2 diabetes and chronic hypertension and/or nephropathy who were treated with metformin. Therefore, with the aim of reducing SGA, it is reasonable to be cautious in our use of metformin in those with type 2 diabetes and chronic hypertension or nephropathy in pregnancy.

This article contains supplementary material online at https://doi.org/10.2337/figshare.19657995.


A complete list of MiTy Collaborative Group members can be found in the appendix.

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