Summary Background Although metformin is increasingly being used in women with type 2 diabetes during pregnancy, little data exist on the benefits and harms of metformin use on pregnancy outcomes in these women. We aimed to investigate the effects of the addition of metformin to a standard regimen of insulin on neonatal morbidity and mortality in pregnant women with type 2 diabetes. Methods In this prospective, multicentre, international, randomised, parallel, double-masked, placebo-controlled trial, women with type 2 diabetes during pregnancy were randomly assigned from 25 centres in Canada and four in Australia to receive either metformin 1000 mg twice daily or placebo, added to insulin. Randomisation was done via a web-based computerised randomisation service and stratified by centre and pre-pregnancy BMI ( ClinicalTrials.gov , NCT01353391 . Findings Between May 25, 2011, and Oct 11, 2018, we randomly assigned 502 women, 253 (50%) to metformin and 249 (50%) to placebo. Complete data were available for 233 (92%) participants in the metformin group and 240 (96%) in the placebo group for the primary outcome. We found no significant difference in the primary composite neonatal outcome between the two groups (40% vs 40%; p=0·86; relative risk [RR] 1·02 [0·83 to 1·26]). Compared with women in the placebo group, metformin-treated women achieved better glycaemic control (HbA1c at 34 weeks' gestation 41·0 mmol/mol [SD 8·5] vs 43·2 mmol/mol [–10]; 5·90% vs 6·10%; p=0·015; mean glucose 6·05 [0·93] vs 6·27 [0·90]; difference −0·2 [–0·4 to 0·0]), required less insulin (1·1 units per kg per day vs 1·5 units per kg per day; difference −0·4 [95% CI −0·5 to −0·2]; p Interpretation We found several maternal glycaemic and neonatal adiposity benefits in the metformin group. Along with reduced maternal weight gain and insulin dosage and improved glycaemic control, the lower adiposity and infant size measurements resulted in fewer large infants but a higher proportion of small-for-gestational-age infants. Understanding the implications of these effects on infants will be important to properly advise patients who are contemplating the use of metformin during pregnancy. Funding Canadian Institutes of Health Research, Lunenfeld-Tanenbaum Research Institute, University of Toronto.
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