title: Trends in twin preterm birth subtypes in the United States, 1989 through 2000: impact on perinatal mortality.
abstract: OBJECTIVE
  We examined trends in twin preterm birth <37 weeks following ruptured membranes (ROM), medically indicated preterm birth, and preterm birth following spontaneous onset of labor (PTL). We further examined whether the changes in preterm birth subtypes were associated with trends in twin perinatal mortality.
  
  
  METHODS
  We carried out a retrospective cohort study of 1,172,405 twin live births and stillbirths delivered in the US between 1989 and 2000. Trends in preterm birth subtypes and perinatal mortality (stillbirths at > or = 22 weeks plus neonatal deaths within 28 days) were examined through ecological logistic regression models after adjusting for confounders.
  
  
  RESULTS
  Twin preterm birth among whites increased from 46.6% in 1989 to 1990 to 56.7% in 1999 to 2000, and from 56.1% to 61.0% among blacks over the same period. Medically indicated preterm birth increased by 50% (95% CI 49-52) among whites, and by 33% (95% CI 29-36) among blacks. PTL increased by 24% among whites, but remained fairly unchanged among blacks between the two periods. Preterm birth following ROM also did not change between the 2 periods among whites, but declined by 7% among blacks. Perinatal mortality among twin births declined by 41% (95% CI 38-44) among whites, and by 37% (95% CI 32-42) among blacks between 1989 and 1990 and 1999 and 2000. This mortality decline was most closely associated with the increase in medically indicated preterm birth among whites, and with the decrease in preterm birth following ROM among blacks.
  
  
  CONCLUSIONS
  Temporal trends in twin preterm birth varied substantially based on underlying subtypes and race. The increase in medically indicated preterm birth is associated with a large reduction in perinatal mortality.
authors.affiliation: Division of Epidemiology and Biostatistics, Department of Obstetrics, Gynecology, and Reproductive Sciences, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, New Brunswick, NJ 08901-1977, USA. cande.ananth@umdnj.edu
authors.forename: Cande V
authors.initials: CV
authors.lastname: Ananth
authors.forename: K S
authors.initials: KS
authors.lastname: Joseph
authors.forename: Kitaw
authors.initials: K
authors.lastname: Demissie
authors.forename: Anthony M
authors.initials: AM
authors.lastname: Vintzileos
mesh_terms: D000328:Adult; D001741:Black or African American / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D007226:Infant Mortality / Q000639:trends*; D007231:Infant, Newborn; D007234:Infant, Premature; D008423:Maternal Age; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000706:statistics & numerical data*; D047928:Premature Birth / Q000453:epidemiology*; D012189:Retrospective Studies; D014427:Twins; D014481:United States / Q000453:epidemiology; D044465:White People / Q000706:statistics & numerical data