Background Previous reviews examining the effect of participation in trials on outcomes have not consistently shown benefit. Obstetrics and gynaecology is a unique disease area posing challenges for both researchers and patients.Objectives To determine whether participation in randomised controlled trials (RCTs), compared with non-participation, has a beneficial effect on women's health.Search strategy Medline, Embase, the Cochrane Library, and PsycInfo were searched up to December 2015.Selection criteria We selected studies that reported the same clinical outcomes for participants in a women's health RCT and a comparable non-participant cohort.Data collection and analysis Data were extracted on quality, characteristics and study results. Outcomes were compared using logistic regression.Main results There were 21 relevant studies (20 160 women, 4759 outcome events). Trial participants, compared with nonparticipants, had 25% better odds of improved outcomes on average (OR 0.75; 95% CI 0.64-0.87; I 2 = 64.3%). The beneficial effect of participating in a trial was larger in comparisons where: RCTs were of high quality (OR 0.62; 95% CI 0.50-0.76) versus low (OR 0.92; 95% CI 0.74-1.16); and RCT intervention was not available to non-participants (OR 0.57; 95% CI 0.47-0.69) versus when it was (OR 1.13; 95% CI 0.89-1.44). The effect of trial participation was not influenced by effect size within the RCT (P = 0.48), whether funding was received or not (P = 0.13), whether non-participants received any treatment or not (P = 0.49), and the quality of the comparison of RCT participants with non-participants (P = 0.88).Conclusions Women participating in RCTs on average experienced better outcomes compared with those outside trials.Keywords Neonate, randomised, systematic review, trial participation, women's health.Tweetable abstract Participants in obstetric and gynaecology RCTs experience better outcomes compared with nonparticipants.Please cite this paper as: Nijjar SK, D'Amico MI, Wimalaweera NA, Cooper NAM, Zamora J, Khan KS. Participation in clinical trials improves outcomes in women's health: a systematic review and meta-analysis.