title: Mid-trimester endovaginal sonography in women at high risk for spontaneous preterm birth.
abstract: BACKGROUND
  Although shortened cervical length has been consistently associated with spontaneous preterm birth, it is not known when in gestation this risk factor becomes apparent.
  
  
  OBJECTIVE
  To determine whether sonographic cervical findings between 16 weeks' and 18 weeks 6 days' gestation predict spontaneous preterm birth and whether serial evaluations up to 23 weeks 6 days' gestation improve prediction in high-risk women.
  
  
  METHODS
  Blinded observational study performed between March 1997 and November 1999 at 9 university-affiliated medical centers in the United States in 183 women with singleton gestations who previously had experienced a spontaneous birth before 32 weeks' gestation.
  
  
  METHODS
  Certified sonologists performed 590 endovaginal sonographic examinations at 2-week intervals. Cervical length was measured from the external os to the functional internal os along a closed endocervical canal. Funneling and dynamic cervical shortening were also recorded.
  
  
  METHODS
  Spontaneous preterm birth before 35 weeks' gestation, analyzed by selected cutoff values of cervical length.
  
  
  RESULTS
  Forty-eight women (26%) experienced spontaneous preterm birth before 35 weeks' gestation. A cervical length of less than 25 mm at the initial sonographic examination was associated with a relative risk (RR) for spontaneous preterm birth of 3.3 (95% confidence interval [CI], 2.1-5.0; sensitivity = 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 evaluations, after any dynamic shortening, the RR of a cervical length of less than 25 mm for spontaneous preterm birth increased to 4.5 (95% CI, 2.7-7.6; sensitivity = 69%; specificity = 80%; positive predictive value = 55%). Compared with 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 prediction of spontaneous preterm birth in a receiver operating characteristic curve analysis (P =.03).
  
  
  CONCLUSIONS
  Cervical length assessed by endovaginal sonography between 16 weeks' and 18 weeks 6 days' gestation, augmented by serial evaluations, predicts spontaneous preterm birth before 35 weeks' gestation in high-risk women.
authors.affiliation: Department of Obstetrics and Gynecology, University of Alabama at Birmingham, 619 19th St S, OHB 458, Birmingham, AL 35249-7333, USA. johnowen@uab.edu
authors.forename: J
authors.initials: J
authors.lastname: Owen
authors.forename: N
authors.initials: N
authors.lastname: Yost
authors.forename: V
authors.initials: V
authors.lastname: Berghella
authors.forename: E
authors.initials: E
authors.lastname: Thom
authors.forename: M
authors.initials: M
authors.lastname: Swain
authors.forename: G A
authors.initials: GA
authors.lastname: Dildy
authors.forename: M
authors.initials: M
authors.lastname: Miodovnik
authors.forename: O
authors.initials: O
authors.lastname: Langer
authors.forename: B
authors.initials: B
authors.lastname: Sibai
authors.forename: D
authors.initials: D
authors.lastname: McNellis
mesh_terms: D000328:Adult; D002584:Cervix Uteri / Q000000981:diagnostic imaging* / Q000473:pathology; D019160:Endosonography; D005260:Female; D006801:Humans; D007752:Obstetric Labor, Premature; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D018566:Pregnancy, High-Risk; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal