The study analyzed data from 104 randomized trials with 35,993 women, including IPD from 54 trials (24,391 women). Lifestyle interventions reduced the risk of gestational diabetes by 10% by any criteria in the IPD trials (OR 0.90, 95% CI 0.80–1.02; absolute reduction 1.3%, 95% CI −0.3% to 2.6%) and by 20% when combined with other data (OR 0.73–0.88; absolute reduction 2.6%, 95% CI −0.3% to 2.6%). % CI 1.6–3.6%). According to NICE criteria, no reduction was observed (OR 0.98, 95% CI 0.84–1.13), while according to IADPSG the risk decreased by 14% in IPD (OR 0.86, 95% CI 0.75–0.97; absolute 2.7%, 95% CI 0.6–5.0%) and by 18% overall (OR 0.72–0.93; absolute 3.5%, 95% CI 1.3–5.7%). Effects did not differ by BMI, age, parity, or ethnicity, but women with low education had a smaller benefit (low vs. medium interaction: OR 0.68, 95% CI 0.51–0.90). The group format (OR 0.81, 95% CI 0.68–0.97; absolute 2.5%, 95% CI 0.4–4.3%) and newly trained facilitators were more effective. Physical activity-based interventions were ranked highest (mean rating 1.1, 95% CI 1–2).