Second-trimester transvaginal ultrasound measurement of cervical length for prediction of preterm birth: a blinded prospective multicentre diagnostic accuracy study

Objective To estimate the diagnostic performance of sonographic cervical length for the prediction of preterm birth (PTB). Design Prospective observational multicentre study. Setting Seven Swedish ultrasound centres. Sample A cohort of 11 456 asymptomatic women with a singleton pregnancy. Methods Cervical length was measured with transvaginal ultrasound at 18-20 weeks of gestation (Cx1) and at 21-23 weeks of gestation (Cx2, optional). Staff and participants were blinded to results. Main outcome measures Area under receiver operating characteristic curve (AUC), sensitivity, specificity, positive and negative predictive values (PPV and NPV), positive and negative likelihood ratios (LR+ and LR-), number of false-positive results per true-positive result (FP/TP), number needed to screen to detect one PTB (NNS) and prevalence of 'short' cervix. Results Spontaneous PTB (sPTB) at <33 weeks of gestation occurred in 56/11 072 (0.5%) women in the Cx1 population (89% white) and in 26/6288 (0.4%) in the Cx2 population (92% white). The discriminative ability of shortest endocervical length was better the earlier the sPTB occurred and was better at Cx2 than at Cx1 (AUC to predict sPTB at <33 weeks of gestation 0.76 versus 0.65, difference in AUC 0.11, 95% CI 0.01-0.23). At Cx2, the shortest endocervical length of <= 25 mm (prevalence 4.4%) predicted sPTB at <33 weeks of gestation with sensitivity 38.5% (10/26), specificity 95.8% (5998/6262), PPV 3.6% (10/274), NPV 99.7% (5988/6014), LR+ 9.1, LR- 0.64, FP/TP 26 and NNS 629. Conclusions Second-trimester sonographic cervical length can identify women at high risk of sPTB. In a population of mainly white women with a low prevalence of sPTB its diagnostic performance is at best moderate. Tweetable abstract Cervical length screening to predict preterm birth in a white low-risk population has moderate performance.

Automated summary

The study indicates that second-trimester sonographic cervical length can identify women at high risk of spontaneous preterm birth. However, in a population of mainly white women with a low prevalence of preterm birth, its diagnostic performance is at best moderate.