OBJECTIVES: To assess cervical length (CL) longitudinally in the first and second trimester and to determine the proportion of women with a short CL. Further, to assess if women with a short CL at 19-24 weeks could be identified at the combined first-trimester screening (cFTS) at 11-14 weeks in relation to a potential implementation of CL screening in the Danish population.METHODS: We recruited singleton pregnant women attending the combined first-trimester screening from 1 November 2013 to 1 December 2014 to a longitudinal prospective study at three University Hospitals in Denmark. We excluded women with multiple pregnancies, uterine anomalies, cerclage, or progesterone treatment at inclusion. CL was measured by transvaginal sonography at 11-14 weeks (Cx1), 19-21 weeks (Cx2) and 23-24 weeks (Cx3). CL was measured as a straight line from the external to internal os by trained operators. Women with a CL≤25 mm were referred to a maternal fetal medicine specialist for treatment according to a standardized management protocol.RESULTS: Of 4904 eligible women, 3477 (71%) women participated and underwent CL measurement at the 11-14 weeks scan (Cx1). Of those, 3232 (93.6%) women had a CL measured at all three time points. The median CL was 37 mm at Cx1 and 40 mm at Cx2 and Cx3. The proportion of women with CL≤25 mm increased with increasing gestational age; 0.41% (95% CI 0.19-0.62) at Cx1 to 1.79% (95% CI 1.34-2.24) at Cx3. In total, the proportion of women with a second-trimester CL≤25 mm (Cx2/Cx3) was 2.0% (n=67 cases) of which 38.8% (n=26 cases) were detected at 19-21 weeks. The probability of short CL between 19-24 weeks increased the shorter the first-trimester CL, nearly nine-fold higher for women with Cx1≤25mm compared to Cx1≥35mm (17% vs. 2%). The performance of Cx1 to predict short CL in the second trimester was 50% at a 10% false-positive rate. More than 1500 women would need to be CL screened at 19-21 weeks to prevent one case of spontaneous preterm delivery before 34 weeks in a population like ours.CONCLUSION: Our data showed an association between first-trimester CL and the risk of short cervix in the second trimester. Once a short CL was observed the risk of preterm delivery (PTD) was highly increased. Whether universal CL screening should be implemented in our low risk population must however depend on a cost-benefit analysis taking into account the low proportions of women with short CL and of PTD.