Cervical length at 16-22 weeks' gestation and risk for preterm delivery

Objective: To determine whether cervical length, as measured by transvaginal sonogram in asymptomatic gravidas at 16-22 weeks, is associated with risk of spontaneous preterm delivery. Methods: In a prospective observational study in an unselected urban tertiary care population, cervical length was measured by transvaginal ultrasound during routine anatomic surveys in 760 gravidas at 16 0/7 to 22 6/7 weeks. The predictor variable was cervical length and the outcome variable was gestational age at delivery. Care providers were not masked to the results. Spontaneous preterm delivery was analyzed as before 37, 35, and 32 weeks. Shortened cervical length was defined by the tenth, fifth, and two and a half percentiles for our population. Yates-corrected chi (2) was used to evaluate the significance on univariate analysis of the relative risk (RR) and 95% confidence intervals (CI). Multiple logistic regression analysis was used to control for background variables in evaluating the probability of preterm delivery at less than 35 weeks. Sensitivity, specificity, positive and negative predictive values also were calculated. Results: Cervical lengths were normally distributed (mean 38.5 +/- 8.0 mm at 19.9 +/- 1.5 weeks) independent of gestational age at measurement, and the tenth, fifth, and two and a half percentiles were 30, 27, and 22 mm, respectively. Eighty-five women delivered before 37 weeks, 51 before 35 weeks, and 27 before 32 weeks. Relative risks (95% CI) for spontaneous preterm delivery before 37 weeks were 3.8 (2.6, 5.6), 5.4 (3.3, 9.0), and 6.3 (3.0, 13.0) for the tenth, fifth, and two and a half percentiles, respectively; RRs for before 35 weeks were 4.5 (2.9, 6.9), 7.5 (4.5, 12.5), and 7.8 (3.6, 16.7); and for before 32 weeks were 5.2 (3.3, 8.3), 9.7 (5.8, 16.1), and 8.4 (3.6, 19.9), respectively. Multiple logistic regression analysis confirmed that cervical length was a significant predictor of preterm birth before 35 weeks, and that paras had a 43% greater risk compared with nulliparas. Sensitivity ranged from 13-44%, specificity 90-99%, positive predictive value 15-47%, and negative predictive value 80-98%. Conclusion: Transvaginal measurement of cervical length during routine ultrasound at 16-22 weeks' gestation in asymptomatic gravidas might help identify women at risk for spontaneous preterm delivery. (Obstet Gynecol 2000;96: 972-8. (C) 2000 by The American College of Obstetricians and Gynecologists.).