title: Cervical length at mid-gestation in screening for preterm birth in twin pregnancy.
abstract: OBJECTIVE
  Short cervical length (CL) in mid-gestation is considered predictive of spontaneous preterm birth (PTB). The medical literature suggests 20 mm as the cut-off for high risk in twin pregnancies. Our objective was to assess the predictive value of CL for spontaneous PTB < 32 weeks' gestation in twin pregnancies and to calculate the cut-off point with the best sensitivity and specificity.
  
  
  METHODS
  This was a single-center retrospective cohort study of women in whom CL had been measured by transvaginal ultrasound at 18-23 weeks' gestation. Pregnancies complicated by twin-to-twin transfusion syndrome, those requiring intrauterine therapy or those with indicated PTB were excluded. The predictive value of CL for PTB < 32 weeks was assessed. The distribution of CL measurements and the optimal cut-off in patients with PTB were calculated and logistic regression analysis was performed to assess the association between pregnancy characteristics and PTB.
  
  
  RESULTS
  A total of 940 twin pregnancies were included. CL showed an area under the receiver-operating characteristics curve of 0.65 (95% CI, 0.58-0.71) for the prediction of PTB < 32 weeks. The optimal cut-off value for predicting PTB was 36 mm (sensitivity, 64.1%; specificity, 62.8%; positive predictive value, 13.5%; negative predictive value, 95.1%; accuracy, 62.9%). The relative risk of PTB with CL ≤ 36 mm was 2.35 (95% CI, 1.53-3.60; P < 0.001). After adjusting for confounders in logistic regression analysis, only CL (adjusted odds ratio (aOR), 0.94 (95% CI, 0.90-0.99); P = 0.03), and not monochorionicity (aOR 4.14 (95% CI, 0.89-19.25); P = 0.07), was independently associated with PTB. More than one-third (36%) of PTB cases delivering < 32 weeks had a normal CL in mid-gestation. This proportion rose to 85% when considering the 20-mm cut-off suggested by the medical literature.
  
  
  CONCLUSIONS
  This study shows that, despite the weak independent association, CL assessed in mid-gestation is a poor predictor of PTB < 32 weeks' gestation in asymptomatic twin pregnancies. Copyright © 2015 ISUOG. Published by John Wiley & Sons Ltd.
authors.affiliation: Maternal-Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Spedali Civili and University of Brescia, Brescia, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Pagani
authors.affiliation: Maternal-Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Spedali Civili and University of Brescia, Brescia, Italy.
authors.forename: V
authors.initials: V
authors.lastname: Stagnati
authors.affiliation: Maternal-Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Spedali Civili and University of Brescia, Brescia, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Fichera
authors.affiliation: Maternal-Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Spedali Civili and University of Brescia, Brescia, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Prefumo
mesh_terms: D000328:Adult; D054791:Cervical Length Measurement; D002584:Cervix Uteri / Q000000981:diagnostic imaging*; D015331:Cohort Studies; D005260:Female; D006801:Humans; D011237:Predictive Value of Tests; D011247:Pregnancy; D011256:Pregnancy Outcome; D011262:Pregnancy Trimester, Second; D059285:Pregnancy, Twin; D047928:Premature Birth / Q000000981:diagnostic imaging* / Q000517:prevention & control; D012372:ROC Curve; D012189:Retrospective Studies; D014463:Ultrasonography
keywords: CL; PTB; cervical length; preterm birth; risk; twins