Summary: In a placebo-controlled multicentre trial in type 1 diabetes pregnancy, adjunctive metformin did not significantly change the primary outcome of total insulin requirement, but did reduce the rise in mealtime insulin and kept gestational weight gain within target more often, with effects strongest in insulin-resistant subgroups.
PICO Summary
| Element | Detail |
|---|---|
| Population | 126 pregnant women with type 1 diabetes (101 completed; target of 200 not reached); double-blind, multicentre, Finland. |
| Intervention | Metformin as an adjunct to insulin. |
| Comparison | Placebo with standard insulin therapy. |
| Outcome | Primary outcome (change in total insulin) not significant (33 vs 27 IU; p=0.193). Prandial insulin rise was lower with metformin (24 vs 14 IU; p=0.014), and weight gain stayed within target more often (20% vs 40%; p=0.029), with larger effects in high-BMI or high-insulin-requirement subgroups. No difference in glycaemic control or neonatal outcomes. |
Metformin in type 1 diabetes pregnancy
RCT · type 1 diabetes pregnancy · multicentre
Adjunctive metformin did not reduce total insulin requirement (the primary outcome) but blunted the mealtime insulin rise and kept gestational weight gain within target more often, with the largest effects in insulin-resistant women.
Expert Commentary
This is an honest, appropriately cautious trial whose headline must start with the negative: metformin did not significantly reduce total insulin requirement, the prespecified primary outcome, so on its own terms the study did not meet its aim. What it did show is more nuanced and plausible: a reduction in the rise of mealtime insulin and better control of gestational weight gain, both concentrated in women with markers of insulin resistance such as higher BMI or already high insulin needs. That pattern fits the mechanism, metformin improves insulin sensitivity and blunts post-meal hepatic glucose output, so it helps where resistance exists rather than across all type 1 pregnancies. Important caveats temper enthusiasm: the trial was underpowered, missing its recruitment target; glycaemic control and neonatal outcomes did not differ; and the subgroup findings are exploratory. Metformin also crosses the placenta, so long-term offspring data matter. Can I use this with my patients? Selectively. It supports considering adjunctive metformin in a pregnant type 1 patient who is overweight or markedly insulin-resistant and gaining excess weight, as an adjunct to intensive insulin, while being clear it did not reduce total insulin or improve glycaemic control overall.
References
Juuma E, Tihtonen K, Metso SE, et al. The effect of metformin on insulin requirement, glycaemic control and weight gain in type 1 diabetes during pregnancy—a randomised, placebo-controlled multicentre study. Diabetes Metab Res Rev. 2025;41(6):e70085. doi:10.1002/dmrr.70085