To Örebro University

OBJECTIVE: To estimate the diagnostic performance of sonographic cervical length for prediction of preterm birth (PTB).

DESIGN: Prospective observational multicentre study.

SETTING: Seven Swedish ultrasound centres.

SAMPLE: 11456 asymptomatic women with a singleton pregnancy.

METHODS: Cervical length was measured with transvaginal ultrasound at 18 to 20 weeks (Cx1) and at 21 to 23 weeks (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, NPV), positive and negative likelihood ratios (LR+, LR-), number of false positive results per true positive result (FP/TP), number needed to screen to detect one PTB (NNS), prevalence of "short" cervix.

RESULTS: Spontaneous PTB (sPTB) <33 weeks occurred in 56/11072 (0.5%) women in the Cx1 population (89% white ethnicity) and in 26/6288 (0.4%) in the Cx2 population (92% white ethnicity). The discriminative ability of shortest endocervical length was better the earlier the sPTB occurred and better at Cx2 than at Cx1 (AUC to predict sPTB <33 weeks 0.76 versus 0.65, difference in AUC 0.11, 95% CI 0.01 to 0.23). At Cx2, shortest endocervical length ≤25 mm (prevalence 4.4%) predicted sPTB <33 weeks 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, 26 FP/TP, 629 NNS.

CONCLUSION: Second trimester sonographic cervical length can identify women at high risk of sPTB. In a population of mainly white women and low prevalence of sPTB its diagnostic performance is at best moderate.