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 (C×1) and at 21–23 weeks of gestation (C×2, 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 C×1 population (89% white) and in 26/6288 (0.4%) in the C×2 population (92% white). The discriminative ability of shortest endocervical length was better the earlier the sPTB occurred and was better at C×2 than at C×1 (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 C×2, 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.