AbstractObjectivesPrevious studies on singleton pregnancies have indicated that progestogens may reduce the rate of cervical shortening during pregnancy. The aim of this study was to investigate whether treatment with 17‐alpha hydroxyprogesterone caproate (17‐OHPC) has an effect on cervical shortening in twin pregnancies.MethodsThis was a secondary analysis of patients who had participated in a multicenter randomized clinical trial on the effectiveness of 17‐OHPC in preventing preterm birth in multiple pregnancies (the AMPHIA‐trial). We included all trial participants with a twin gestation who had undergone repeat cervical length measurements during pregnancy. We performed a separate analysis of women with repeat measurements in centers where this was standard protocol for multiple pregnancies. The rate of cervical shortening for both the 17‐OHPC group and the placebo group was analyzed using a linear mixed model.ResultsOf the 671 patients who participated in the trial, 282 (42%) had a twin pregnancy and underwent two or more cervical length measurements. Of these women, 140 were monitored in centers where repeat measurements were standard protocol. We observed an overall reduction of cervical length from 44.3 mm at 14–18 weeks to 30.0 mm at 30–34 weeks' gestation. In the 17‐OHPC group, cervical length decreased by 1.04 mm each gestational week, while this was 1.11 mm per week for the placebo group (P = 0.6). For the overall group, each 10% decrease in cervical length led to an increase in the risk of preterm birth (hazard ratio, 1.14; 95% CI, 1.08–1.21).ConclusionIn women with a twin pregnancy, there is progressive shortening of the cervix during pregnancy, regardless of 17‐OHPC use. Copyright © 2012 ISUOG. Published by John Wiley & Sons, Ltd.