Progesterone is critical for the maintenance of pregnancy. Several small studies have suggested that progesterone supplementation may reduce the risk of miscarriage in women with recurrent miscarriage. However, given the uncertainty in the evidence, the effect of first trimester use of vaginal micronised progesterone was recently evaluated in a large high-quality multicentre placebo-controlled trial, called the PROMISE trial, which randomised 836 women from 45 hospitals in the UK and the Netherlands. The PROMISE trial found a 3% higher live birth rate with progesterone, but with substantial statistical uncertainty. However, when all the studies of progesterones were combined in a meta-analysis, a clear benefit in favour of progesterone for the outcome of live birth or ongoing pregnancy was identified (RR = 1.08, 95% CI 1.03 to 1.14, I2 = 0%). A key finding, first observed in the PROMISE trial, and then replicated in another large high-quality trial of progesterone in women with early pregnancy bleeding, called the PRISM trial, was that treatment with vaginal micronised progesterone was associated with increasing live birth rates according to the number of previous miscarriages. No short-term safety concerns were identified from the PROMISE and PRISM trials. Women and care providers should use these findings for shared decision-making.