title: Natural history of placenta previa in twins.
abstract: OBJECTIVE
  To estimate the incidence of placenta previa in twin pregnancies compared with singletons and to estimate the rate and gestational age of previa resolution in twin pregnancies.
  
  
  METHODS
  This was a retrospective cohort of singleton and twin pregnancies undergoing ultrasonography at 15-22 weeks of gestation and of twin pregnancies undergoing serial ultrasonography from 15 to 40 weeks of gestation. Groups were defined by singleton or twin gestation and by chorionicity of twin gestation. The primary outcomes were incidence of placenta previa in each group and the percentage of all previa resolving at 24-28, 28-32, 32-36, and 36 or more weeks of gestation.
  
  
  RESULTS
  Of 67,895 pregnancies included, 2.1% (1,381 of 65,701) of singleton and 2.5% (56 of 2,194) of twin pregnancies had previa diagnosed (P=.15). Dichorionic twins had an increased risk of placenta previa compared with singletons (adjusted odds ratio 1.54, 95% confidence interval 1.15-2.06) or monochorionic twin pregnancies (relative risk 3.29, 95% confidence interval 1.32-8.21). Of the 1,738 twin pregnancies with serial ultrasound examinations, 51 (2.9%) were noted to have previa. Sixty-nine percent of the previa resolved by 32 weeks, at between 32 and 36 weeks an additional 47% of the remaining previa resolved, and no previa resolved after 36 weeks.
  
  
  CONCLUSIONS
  Dichorionic twin pregnancies are at significantly increased risk for ultrasound-diagnosed previa when compared with singleton or monochorionic pregnancies. Among twin pregnancies with previa diagnosed in the second trimester, the majority of cases resolve by 32 weeks.
  
  
  METHODS
  II.
authors.affiliation: Department of Obstetrics and Gynecology, Washington University in St. Louis, St. Louis, Missouri 63110, USA. weism@wudosis.wustl.edu
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Weis
authors.forename: Lorie M
authors.initials: LM
authors.lastname: Harper
authors.forename: Kimberly A
authors.initials: KA
authors.lastname: Roehl
authors.forename: Anthony O
authors.initials: AO
authors.lastname: Odibo
authors.forename: Alison G
authors.initials: AG
authors.lastname: Cahill
mesh_terms: D000328:Adult; D015331:Cohort Studies; D005260:Female; D006801:Humans; D015994:Incidence; D016015:Logistic Models; D008923:Missouri / Q000453:epidemiology; D015999:Multivariate Analysis; D010923:Placenta Previa / Q000000981:diagnostic imaging / Q000453:epidemiology / Q000209:etiology*; D011247:Pregnancy; D059285:Pregnancy, Twin; D012075:Remission, Spontaneous; D012189:Retrospective Studies; D012307:Risk Factors; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic; D016216:Ultrasonography, Prenatal