Hermans FJ Karolinski A Othenin-Girard V Bertolino MV Schuit E Salgado P Hösli I Irion O Laterra C Mol BW Martinez de Tejada B 2016 OBJECTIVE Threatened preterm labor (tPTL) is a complication of pregnancy. Identification of women and clinical definition differs between countries. This study investigated differences in tPTL and effectiveness of vaginal progesterone to prevent preterm birth (PTB) between two countries. METHODS Secondary analysis of a randomized controlled trial (RCT) from Argentina and Switzerland comparing vaginal progesterone to placebo in women with tPTL (n = 379). Cox proportional hazards analysis was performed to compare placebo groups of both countries and to compare progesterone to placebo within each country. We adjusted for baseline differences. Iatrogenic onset of labor or pregnancy beyond gestational age of interest was censored. RESULTS Swiss and Argentinian women were different on baseline. Risks for delivery <14 days and PTB < 34 and < 37 weeks were increased in Argentina compared to Switzerland, HR 3.3 (95% CI 0.62-18), 54 (95% CI 5.1-569) and 3.1 (95% CI 1.1-8.4). In Switzerland, progesterone increased the risk for delivery <14 days [HR 4.4 (95% CI 1.3-15.7)] and PTB <37 weeks [HR 2.5 (95% CI 1.4-4.8)], in Argentina there was no such effect. CONCLUSION In women with tPTL, the effect of progesterone may vary due to population differences. Differences in populations should be considered in multicenter RCTs. https://sonar.ch/global/documents/201912 eng info:eu-repo/semantics/altIdentifier/doi/10.3109/14767058.2015.1121476 info:eu-repo/semantics/altIdentifier/pmid/26586448 The journal of maternal-fetal & neonatal medicine : the official journal of the European Association of Perinatal Medicine, the Federation of Asia and Oceania Perinatal Societies, the International Society of Perinatal Obstetricians. - 2016 Harm population differences preterm birth randomized controlled trial secondary analysis selection criteria threatened preterm labor treatment effect treatment interaction vaginal progesterone Adult Argentina Chi-Square Distribution Double-Blind Method Female Gestational Age Humans Obstetric Labor, Premature Pregnancy Premature Birth Progesterone Progestins Proportional Hazards Models Statistics, Nonparametric Switzerland Young Adult Population differences and the effect of vaginal progesterone on preterm birth in women with threatened preterm labor (.). http://purl.org/coar/resource_type/c_6501