The rate of short cervical length of ≤25 mm was lower than expected. The study confirmed the increased risk of spontaneous preterm delivery in women with a short cervix, although the analysis was based on only a few cases. In Sweden, a larger study is needed to evaluate the prevalence of short cervical length and the possible association with preterm delivery before universal screening can be recommended.

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 (C91) and at 21-23 weeks of gestation (C92, 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 LRÀ), 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 C91 population (89% white) and in 26/6288 (0.4%) in the C92 population (92% white). The discriminative ability of shortest endocervical length was better the earlier the sPTB occurred and was better at C92 than at C91 (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 C92, 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, LRÀ 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.

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Introduction:The aim of the study is to compare the effect of cervical length measured with transvaginal ultrasound in the second trimester on the risk of spontaneous preterm delivery between different risk groups of asymptomatic women with a singleton pregnancy. Material and methods:This is a pre-planned exploratory analysis of the CERVIX study, a prospective blinded multicenter diagnostic accuracy study. Asymptomatic women with a singleton pregnancy were consecutively recruited at their second-trimester routine ultrasound examination at seven Swedish ultrasound centers. Cervical length was measured with transvaginal ultrasound at 18-20 weeks (Cx1; n=11 072) and 21-23 weeks (Cx2, optional; n=6288). The effect of cervical length on the risk of spontaneous preterm delivery and its discriminative ability was compared between women with: i. previous spontaneous preterm delivery, late miscarriage or cervical conization (high-risk group; n=1045); ii. nulliparae without risk factors (n=5173); iii. parae without risk factors (n=4740). Women with previous indicated preterm delivery were excluded (n=114). Main outcome measures were: effect of cervical length on the risk of spontaneous preterm delivery expressed as odds ratio per 5 mm decrease in cervical length with interaction analysis using logistic regression to test if the effect differed between groups, area under the receiver operating characteristic curve (AUC), sensitivity, specificity, number needed to screen to detect one spontaneous preterm delivery. ResultsThe effect of cervical length at Cx2 on the risk of spontaneous preterm delivery <33 weeks was similar in all groups (odds ratios 2.26 to 2.58, interaction P-value 0.91). The discriminative ability at Cx2 was superior to that at Cx1 and was similar in all groups (AUC 0.69 to 0.76). Cervical length ≤25 mm at Cx2 identified 57% of spontaneous preterm deliveries <33 weeks in the high-risk group with number needed to screen 161. The number needed to screen for groups ii and iii were 1018 and 843. ConclusionsThe effect of cervical length at 21-23 weeks on the risk of spontaneous preterm delivery <33 weeks is similar in high-and low-risk pregnancies. The differences in number needed to screen should be considered before implementing a screening program.