Approximately half of women with a diagnosis of Gestational Diabetes Mellitus (GDM) fail to achieve conventional blood glucose level (BGL) targets using diet and lifestyle modifications alone. Insulin therapy is usually employed in such cases, although recent studies demonstrate that metformin may be efficacious, safe, cheaper and more acceptable to patients.
We performed a retrospective audit of outcomes in 1539 pregnancies complicated by GDM at Blacktown Hospital, Sydney, from 2008 to 2013. All patients were recommended an appropriate diet and lifestyle plan and monitored BGLs, with a target of fasting <5.5mmol/L (99mg/dl) and 2-hour post-prandial <7.0mmol/L (126 mg/dl). Women with BGLs above target were commenced on Metformin monotherapy (n=351) or insulin (n=334) according to the endocrinologist’s choice and/or patient’s preference.
Although treatment was not randomly allocated, there were no significant baseline differences between the Metformin and Insulin groups for age, pre-pregnancy weight, BMI, or ethnicity. BGL control, measured as pre-delivery HbA1c levels, were lower in women initially commenced on metformin (mean 5.61% ± sem 0.02) compared with insulin (5.94% ± 0.03, p<0.001), and both were higher than the diet treated group (5.37% ± 0.01, p<0.001). Maternal weight gain in the interval between GDM diagnosis and delivery was significantly higher in women using insulin alone (mean 3.7kg ± sem 0.23) compared with those on diet treatment only (2.5 kg ± 0.12, p<0.005) or metformin (2.9 kg ± 0.25, p=0.03).
There was no significant difference in birth weight between metformin exposed infants (3240g ± 356) and those in the diet only group (3291g ± 598) or insulin treated group (3348g ± 331). There was no significant effect of metformin exposure on the incidence of major perinatal outcomes, including stillbirth, fetal anomaly, mode of delivery or APGAR score.
In our clinical experience of women with GDM, metformin is well tolerated, associated with at least equivalent glycaemic control and perinatal safety to insulin treatment, but causes less maternal weight gain.