Metformin Versus Insulin and Risk of Major Congenital Malformations in Pregnancies With Type 2 Diabetes: A Nordic Register-Based Cohort Study.

Objective: To assess the risk of major congenital malformations with metformin versus insulin in pregnancies with type 2 diabetes.
Research Design and Methods: This cohort study used four Nordic countries' nationwide registers of live and stillborn infants exposed to metformin or insulin during first trimester organogenesis. Main exclusion criteria were type 1 diabetes, polycystic ovary syndrome, fertility treatment, and exposure to other diabetes drugs. Adjusted risk ratios (RRs) and 95% CIs were estimated for any and cardiac malformations.
Results: Of 3,734,125 infants in the source population, 25,956 were exposed to metformin or insulin in the first trimester, and 4,023 singleton infants were included. A malformation was diagnosed in 147 (4.7%) of 3,145 infants with exposure to any metformin (alone or in addition to insulin) and 50 (5.7%) of 878 infants with exposure to insulin alone (RR 0.84, 95% CI 0.46-1.54). Among 2,852 infants exposed to metformin alone and 293 infants exposed to metformin in addition to insulin 127 (4.4%) and 20 (6.8%), respectively, had a malformation. The adjusted risk was not increased for either metformin alone (0.83, 0.44-1.58) or both metformin and insulin (0.98, 0.56-1.69) versus insulin alone. Corresponding RRs for cardiac malformations were 1.01 (0.55-1.84) for any metformin, 0.92 (0.47-1.81) for metformin alone, and 1.72 (0.76-3.91) for both metformin and insulin.
Conclusions: No evidence of an increased malformation risk with metformin versus insulin in the first trimester was found. Results should be interpreted with caution since information on glycemic control was missing.
(© 2023 by the American Diabetes Association.)