To determine the feasibility of a definitive trial of metformin to prevent type 2 diabetes in the postnatal period in women with gestational diabetes.

A multicentre, placebo-controlled, double-blind randomised feasibility trial with qualitative evaluation.

Three inner-city UK National Health Service hospitals in London.

Pregnant women with gestational diabetes treated with medication.

2 g of metformin (intervention) or placebo (control) from delivery until 1 year postnatally.

Rates of recruitment, randomisation, follow-up, attrition and adherence to the intervention.

Preliminary estimates of glycaemic effects, qualitative exploration, acceptability of the intervention and costs.

Out of 302 eligible women, 57.9% (175/302) were recruited. We randomised 82.3% (144/175) of those recruited, with 71 women in the metformin group and 73 women in the placebo group. Of the participants remaining in the study and providing any adherence information, 54.1% (59/109) took at least 75% of the target intervention dose; the overall mean adherence was 64% (SD 33.6). Study procedures were found to be acceptable to women and healthcare professionals. An increased perceived risk of developing type 2 diabetes, or a positive experience of taking metformin during pregnancy, encouraged participation and adherence to the intervention. Barriers to adherence included disruption to the medication schedule caused by the washout periods ahead of each study visit or having insufficient daily reminders.

It is feasible to run a full-scale definitive trial on the effectiveness of metformin to prevent type 2 diabetes in women with gestational diabetes, during the early postnatal period. Adherence and engagement with the study could be improved with more regular reminders and potentially the addition of ongoing educational or peer support to reinforce messages around type 2 diabetes prevention.

ISRCTN20930880.