Figure 4.

Kaplan-Meier plot

Survival analysis shows the probability of patients continuing pregnancy without spontaneous delivery by days from the cervical length measurement to delivery for the progesterone treatment group (red line) and the no-treatment group (blue line). Progesterone reduces the risk of spontaneous delivery before 37 weeks of pregnancy with an adjusted hazard ratio of 0.50 (95% confidence interval 0.28–0.73; P = 0.001). An adjustment was performed to maternal age, cervical length, gestational age at the diagnosis, and preterm birth history.