Cervical-length measurement using transvaginal sonography (TVS) is an essential part of assessing the risk of preterm delivery. At mid-gestation, it provides a useful method with which to predict the likelihood of subsequent preterm birth in asymptomatic women. In women who present with threatened spontaneous preterm labor, TVS measurement of cervical length can help to distinguish between ‘true’ and ‘false’ spontaneous preterm labor. Additionally, there is some evidence that measurement of the cervix at the 11+0 to 13+6-week scan can help to establish the risk of preterm birth1,2. To et al.3 reported on cervical-length measurement between 22 and 24 weeks’ gestation in 39 000 women with a singleton pregnancy. The cervical length was found to be distributed normally, with a mean length of 36 mm. In about 1% of the women, the length was 15 mm or less. This cut-off is generally used to define the high-risk group in interventional studies4. In most studies focusing on asymptomatic twin pregnancies at 20 to 24 weeks, a cut-off of 25 mm is applied5. Celik et al.6 used cervical-length measurements obtained between 20 and 24 weeks’ gestation, along with maternal history, in more than 58 000 women to create computed risk models for preterm delivery. They compared patients who delivered before 28 weeks, between 28 and 30 weeks, between 31 and 33 weeks, and between 34 and 36 weeks’ gestation. For a 10% false-positive rate, the sensitivities were 81%, 59%, 53% and 29%, respectively. In a Health Technology Assessment report, Honest et al.7 summarized the results of five studies that used cervical-length measurements between 20 and 24 weeks, with cut-offs of 20–30 mm, to predict preterm birth before 34 weeks’ gestation. The resultant positive likelihood ratios ranged from 2.3 for 30 mm to 7.6 for 20 mm.