title: Timing of mid-trimester cervical length shortening in high-risk women.
abstract: OBJECTIVE
  To examine the natural history of cervical length shortening in women who had experienced at least one prior spontaneous preterm birth at between 17+0 and 33+6 weeks' gestation.
  
  
  METHODS
  This was an analysis of prerandomization data from the multicenter Vaginal Ultrasound Cerclage Trial. Serial cervical length was measured by transvaginal sonography in 1014 high-risk women at 16+0 to 22+6 weeks. We performed survival analyses in which the outcome was cervical length shortening<25 mm and data were censored if this did not occur before 22+6 weeks' gestation. The incidence of cervical length shortening and the time to shortening were compared for women whose earliest prior preterm birth was in the mid-trimester, defined as <24 weeks, vs. those at weeks 24-33. Similar comparisons were performed based on each patient's most recent birth history.
  
  
  RESULTS
  Time to cervical length shortening by survival analysis was significantly shorter (hazard ratio (HR)=2.2, P<0.0001) and the relative risk (RR) of shortening significantly higher (RR=1.8, P<0.0001) for women whose earliest prior spontaneous preterm birth was at <24 weeks. A larger effect was observed for women whose most recent birth was at <24 weeks (HR=2.8, P<0.0001; RR=2.1, P<0.0001). The observed hazard ratios remained significant after adjusting for confounders in a multivariable Cox proportional hazards model.
  
  
  CONCLUSIONS
  Women with a prior spontaneous preterm birth at <24 weeks are at a higher risk of cervical shortening, and do so at a higher rate and at an earlier gestational age, than do women with a later preterm birth history.
authors.affiliation: University of Alabama at Birmingham, Department of Biostatistics, Birmingham, Alabama 35249-7333, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Szychowski
authors.forename: J
authors.initials: J
authors.lastname: Owen
authors.forename: G
authors.initials: G
authors.lastname: Hankins
authors.forename: J
authors.initials: J
authors.lastname: Iams
authors.forename: J
authors.initials: J
authors.lastname: Sheffield
authors.forename: A
authors.initials: A
authors.lastname: Perez-Delboy
authors.forename: V
authors.initials: V
authors.lastname: Berghella
authors.forename: D A
authors.initials: DA
authors.lastname: Wing
authors.forename: E R
authors.initials: ER
authors.lastname: Guzman
mesh_terms: D023802:Cerclage, Cervical; D002584:Cervix Uteri / Q000000981:diagnostic imaging*; D019160:Endosonography; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D008423:Maternal Age; D007752:Obstetric Labor, Premature / Q000453:epidemiology / Q000517:prevention & control*; D010298:Parity; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D018566:Pregnancy, High-Risk; D018570:Risk Assessment; D055502:Secondary Prevention; D013997:Time Factors; D016216:Ultrasonography, Prenatal / Q000379:methods*; D002581:Uterine Cervical Incompetence / Q000000981:diagnostic imaging* / Q000453:epidemiology