Mid-trimester endovaginal sonography in women at high risk for spontaneouspreterm birth

Context Although shortened cervical length has been consistently associated with spontaneous preterm birth, it is not known when in gestation this ris k factor becomes apparent. Objective To determine whether sonographic cervical findings between 16 wee ks' and 18 weeks 6 days' gestation predict spontaneous preterm birth and wh ether serial evaluations up to 23 weeks 6 days' gestation improve predictio n in high-risk women. Design, Setting, and Participants Blinded observational study performed bet ween March 1997 and November 1999 at 9 university-affiliated medical center s in the United States in 183 women with singleton gestations who previousl y had experienced a spontaneous birth before 32 weeks' gestation. Observation Certified sonologists performed 590 endovaginal sonographic exa minations at 2-week intervals. Cervical length was measured from the extern al os to the functional internal os along a closed endocervical canal. Funn eling and dynamic cervical shortening were also recorded. Main Outcome Measure Spontaneous preterm birth before 35 weeks' gestation, analyzed by selected cutoff values of cervical length. Results Forty-eight women (26%) experienced spontaneous preterm birth befor e 35 weeks' gestation. A cervical length of less than 25 mm at the initial sonographic examination was associated with a relative risk (RR) for sponta neous preterm birth of 3.3.(95% confidence interval [CI], 2.1-5.0; sensitiv ity=19%; specificity=98%; positive predictive value=75%). After controlling for cervical length, neither funneling (P=.24) nor dynamic shortening (P=. 054) were significant independent predictors of spontaneous preterm birth. However, using the shortest ever observed cervical length on serial evaluat ions, after any dynamic shortening, the RR of a cervical length of less tha n 25 mm for spontaneous preterm birth increased to 4.5 (95% CI, 2.7-7.6; se nsitivity=69%; specificity=80%; positive predictive value=55%). Compared wi th a single cervical measurement at 16 weeks' to 18 weeks 6 days' gestation , serial measurements at up to 23 weeks 6 days significantly improved the p rediction of spontaneous preterm birth in a receiver operating characterist ic curve analysis (P=.03). Conclusions Cervical length assessed by endovaginal sonography between 16 w eeks' and 18 weeks 6 days' gestation, augmented by serial evaluations, pred icts spontaneous preterm birth before 35 weeks' gestation in high-risk wome n.