Transvaginal sonographic evaluation of cervical length in the second trimester of asymptomatic singleton pregnancies, and the risk of preterm delivery.

Objective: To evaluate cervical length in asymptomatic women with singleton pregnancies in the second trimester by means of transvaginal ultrasonography, and to examine the relation between cervical length and spontaneous preterm delivery.

Design: Observational, prospective study.

Setting: A university hospital and a county hospital in Western Sweden.

Population: A total of 2122 asymptomatic women with live singleton pregnancies without fetal anomalies.

Methods: Cervical length was measured at between 16 and 23 weeks of gestation by means of transvaginal ultrasonography. Data were analysed using logistic regression analysis.

Main Outcome Measures: Cervical length in relation to spontaneous preterm delivery <34 weeks (primary outcome) and <37 weeks of gestation (secondary outcome).

Results: Eleven women had a cervical length of ≤25 mm (0.5%) and 73 women had a cervical length of ≤30 mm (3.4%). Spontaneous preterm delivery at <34 weeks occurred in 22/2061 women (1.1%) and at <37 weeks in 87/2061 women (4.2%). There was a significant association between cervical length and spontaneous preterm delivery at <34 weeks (odds ratio 1.78; 95% confidence interval 1.19-2.65 for a decrease of cervical length by 5 mm) but no significant association at <37 weeks.

Conclusions: 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.