Metformin combined with insulin in women with gestational diabetes mellitus: a propensity score-matched study

Aim Metformin use in gestational diabetes (GDM) is a common practice. Although its use in combination with insulin might be advantageous, it was never formally tested. We studied whether combined treatment was associated with better obstetric or neonatal outcomes compared to insulin alone. Methods This is a retrospective study, using the Portuguese National Registry of GDM (2012-2017), of women treated with insulin +/- metformin. Primary endpoints were obstetric and neonatal complications. Secondary endpoints were gestational weight gain (GWG) and insulin dose. A propensity score-matched analysis was performed to balance the distribution of age, BMI, insulin treatment duration, HbA1c, first trimester diagnosis of GDM and previous GDM or macrosomia. Women treated with metformin plus insulin and insulin only were then compared. Results A total of 4034 women were treated with insulin or insulin plus metformin (10.2%). After propensity score matching, we studied two groups of 386 patients. Obstetric and neonatal complications were similar. Women treated with metformin plus insulin had 201 (52.1%) obstetric complications versus 184 (47.7%) in insulin-only group, p = 0.22; and 112 (29.0%) neonatal complications versus 96 (24.9%), p = 0.19. Patients treated with metformin plus insulin had similar GWG, excessive weight gain and insulin dose compared to the insulin-only group. Conclusions Women with GDM treated with insulin plus metformin had similar obstetric and neonatal complications, weight gained and insulin dose compared to those only treated with insulin.