OBJECTIVE: To assess the efficacy of progestogens for maintenance tocolysis in women undelivered after their first preterm labor episode. METHODS: Women with singleton pregnancies between 22 0/7 and 31 6/7 weeks of gestation with arrested preterm labor and a cervical length 25 mm or less at hospital discharge were eligible. Patients with a previous preterm birth were excluded. In a randomized controlled trial conducted in five university hospitals, women were randomized to receive vaginal progesterone (200 mg per day) or intramuscular 17α-hydroxyprogesterone caproate (341 mg per week) or to an observation groups (control group). The primary outcome was the proportion of women with preterm birth at less than 37 weeks of gestation. A sample size of 160 per group (n=480) was planned to compare vaginal progesterone and 17ahydroxyprogesterone caproate groups with those in the control group. The sample size estimation was based on the hypothesis that the risk of experiencing preterm birth in the control group would be 30% and that 17α-hydroxyprogesterone caproate or progesterone would decrease this risk to 15%. A P value of ≤.025 was defined as statistically significant. At planned interim analysis (n=254), the trial was stopped for futility. RESULTS: Between July 2010 and June 2015, 257 women were eligible and 254 were subsequently randomly assigned to vaginal progesterone (n=86), 17α-hydroxyprogesterone caproate (n=87), or observation (n=81). Nineteen (8%) were excluded from the analysis because they either dropped out or information was missing, leaving 235 women available for analysis. Demographic characteristics were similar across groups. The preterm birth rate did not differ significantly between groups: 23% in the 17a-hydroxyprogesterone caproate group, 39% in the vaginal progesterone group, and 22% in the women in the control group (P=.949 for 17a-hydroxyprogesterone caproate compared with the women in the control group and P=.027 for vaginal progesterone compared with women in the control group). CONCLUSION: The use of progestogens for maintenance tocolysis in women with a short cervix did not reduce the rate of preterm birth.