title: Reference range for cervical length in twin pregnancies.
abstract: BACKGROUND
  Twin pregnancies are at increased risk of preterm delivery, and cervical length measurement by transvaginal ultrasound examination has been shown to be an effective method of screening for this complication. The aim of the present study is to establish a reference range for cervical length measurement throughout gestation in twin pregnancies.
  
  
  METHODS
  Prospective study involving 144 nonselected twin pregnancies that delivered after 34 weeks and underwent ultrasound examination every 4 weeks for transvaginal measurement of cervical length. Mean cervical length measurements, 95% confidence intervals for the estimate and 95% prediction intervals were established throughout gestation by linear regression analysis.
  
  
  RESULTS
  There was a significant correlation between cervical length measurements and gestational age [cervical length (mm) = - 0.758 * gestation age (weeks) + 56.69, r=- 0.43, p < 0.0001], and mean cervical length shortened by approximately 0.8 mm per week, from 47 mm at 13 weeks to 32 mm at 32 weeks. The corresponding lower limits for the 95% prediction intervals were 29 and 15 mm, respectively.
  
  
  CONCLUSIONS
  Cervical length shortens gradually throughout gestation in twin pregnancies.
authors.affiliation: Department of Obstetrics, Hospital das Clinicas, São Paulo University Medical School, Rua Dr Enéas de Carvalho Aguiar, 225 São Paulo, SP, Brazil 05403-000.
authors.forename: Mariza M
authors.initials: MM
authors.lastname: Fujita
authors.forename: Maria de Lourdes
authors.initials: Mde L
authors.lastname: Brizot
authors.forename: Adolfo W
authors.initials: AW
authors.lastname: Liao
authors.forename: Tatiana
authors.initials: T
authors.lastname: Bernáth
authors.forename: Luciana
authors.initials: L
authors.lastname: Cury
authors.forename: Jorge D Banduki
authors.initials: JD
authors.lastname: Neto
authors.forename: Marcelo
authors.initials: M
authors.lastname: Zugaib
mesh_terms: D002584:Cervix Uteri / Q000033:anatomy & histology* / Q000000981:diagnostic imaging*; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D007234:Infant, Premature; D007752:Obstetric Labor, Premature; D011247:Pregnancy; D011272:Pregnancy, Multiple; D011446:Prospective Studies; D012016:Reference Values; D012044:Regression Analysis; D012307:Risk Factors; D012680:Sensitivity and Specificity; D014427:Twins; D016216:Ultrasonography, Prenatal