Background: Several studies have demonstrated that metformin treatment in gestational diabetes mellitus (GDM) is safe and effective. The aim of the present study is to compare the maternal characteristics and perinatal outcomes of women with GDM treated with metformin, in comparison to those treated with insulin therapy or diet and lifestyle modification alone.

Methods: A retrospective, case-control study was carried out in women with GDM attending the Specialist Obstetric Clinic at Royal North Shore Hospital between September, 2012 and April, 2016, treated with either metformin (and supplemental insulin where needed), insulin, or diet and lifestyle modification. Allocation to treatment with metformin or insulin was by physician and patient preference.

Results: 74 women taking metformin in GDM pregnancy were matched 1:1 with women receiving insulin therapy or diet and lifestyle modification. Women treated with metformin had a significantly higher fasting glucose level and HbA1c at diagnosis of GDM compared to women managed with diet and lifestyle modification (p < 0.001 and p < 0.01, respectively); and they had a higher BMI in early pregnancy compared to those on insulin or diet and lifestyle modification (p < 0.001). Despite this there was no difference in neonatal birth weight or the incidence of large or small for gestational age neonates. Women managed with diet and lifestyle modification were more likely to deliver at a later gestational age (p < 0.05). In addition, women taking insulin had a higher rate of neonatal hypoglycaemia (p < 0.05). There was no difference in other adverse perinatal outcomes between groups. 33 of 74 women treated with metformin required supplemental insulin therapy. 

Conclusions: Despite their greater BMI, women with metformin-treated GDM did not have an increased risk of adverse perinatal outcomes. Metformin is a useful alternative to insulin in the management of GDM.