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This study aims to evaluate the utility of first trimester cervical ultrasonography in predicting preterm delivery by separate analysis of measurements of cervical and isthmus length.This is a cohort study based on data collected prospectively on singletons between 1 July 2011 and 1 February 2013. Mean cervical, isthmus and cervico-isthmic complex length were measured for deliveries before and after 37 weeks.A total of 1494 pregnancies were analysed, including 51 cases of spontaneous preterm delivery (3.4%). The cervico-isthmic complex in the first trimester was significantly shorter in patients who delivered before term (43.8 mm vs 47.5 mm, p = 0.04). This difference is related to differences in length at the isthmus (10.7 mm vs 14.1 mm, p = 0.005) rather than at the cervix proper (34.5 mm vs 35.0 mm, p = 0.56).Measurement of the cervico-isthmic complex enables detection of a number of patients who will go on to deliver before term. Further studies are necessary to confirm that isthmic length and not cervical length is predictive of preterm delivery. © 2013 John Wiley & Sons, Ltd.