Prevention of preterm delivery with vaginal progesterone in women with preterm labour (4P): randomised double‐blind placebo‐controlled trial

Objective To evaluate the effectiveness of 200 mg of daily vaginal natural progesterone to prevent preterm birth in women with preterm labour. Design Multicentre, randomised, double‐blind, placebo‐controlled trial. Setting Twenty‐nine centres in Switzerland and Argentina. Population A total of 385 women with preterm labour (240/7 to 336/7 weeks of gestation) treated with acute tocolysis. Methods Participants were randomly allocated to either 200 mg daily of self‐administered vaginal progesterone or placebo within 48 hours of starting acute tocolysis. Main outcome measures Primary outcome was delivery before 37 weeks of gestation. Secondary outcomes were delivery before 32 and 34 weeks, adverse effects, duration of tocolysis, re‐admissions for preterm labour, length of hospital stay, and neonatal morbidity and mortality. The study was ended prematurely based on results of the intermediate analysis. Results Preterm birth occurred in 42.5% of women in the progesterone group versus 35.5% in the placebo group (relative risk [RR] 1.2; 95% confidence interval [95% CI] 0.93–1.5). Delivery at