Introduction
Metformin is used to treat gestational diabetes when glucose levels are mildly abnormal. It can be used alone or in combination with insulin.
Metformin is safe in pregnancy
Metformin crosses the placenta, so this explains the Category C classification. Metformin is safe in pregnancy, and this was confirmed by Australian and New Zealand research studies, which showed that the babies were less likely to have low glucose levels than when mothers with GDM were treated with insulin.
When to start it
Metformin is recommended for women with GDM when glucose levels are above target levels on at least 2 occasions in the space of one week. It may occur before pre-breakfast or post-meals.
Insulin would be a better choice in pregnancies where the baby appears small (EFW below 10th percentile), because Metformin interferes with the transplacental provision of nutrients to the fetus.
Dosing in pregnancy
Starting dose: 500mg daily, taken after a meal – either as native Metformin or extended release
Dose increment: 500 mg every 4 to 7 days, depending on tolerance of side effects
Maximum dose: 2000mg per day
Side effects: Nausea, bloating, flatulence, diarrhoea, poor appetite
