Safety of metformin use during pregnancy
The decision to start, continue or stop metformin during pregnancy should be made in consultation with the patient and their specialist.
A randomised controlled trial of metformin in women with type 2 diabetes during pregnancy1 showed that metformin treatment improved glycaemia, reduced insulin requirements, reduced gestational weight gain and reduced the risk of having a large-for-gestational age infant. However, there was an increased risk of having a small-for-gestational age infant.
The long-term effects on the child from metformin treatment during pregnancy are not clear. Concern has been raised about a possible increase in obesity in offspring, but study findings have not been consistent.
For women taking metformin, serum vitamin B12 concentration should be checked before pregnancy. Consider rechecking during pregnancy if metformin is continued throughout the pregnancy.
If metformin has been used throughout pregnancy, treatment should be stopped before induction of or at the onset of spontaneous labour, or when fasting before caesarean section; restart postpartum (see Management of women with pre-existing diabetes during labour).