title: A comparison of risk factors for twin preterm birth in the United States between 1981-82 and 1996-97.
abstract: OBJECTIVE
  This paper examines risk factors for twin preterm birth in 1981-82 and 1996-97 in the United States in order to see if they have changed over time.
  
  
  METHODS
  We studied all U.S. twin births for the years examined (N = 346, 567). Since the gestational age distributions for twins differs from singletons, the risk of preterm birth was examined at <33, 33-34, and 35-36 weeks. Logistic regression was used to examine the contributions of sociodemographic and obstetric factors at each period.
  
  
  RESULTS
  While the <33 week twin preterm rate rose 7% from 1981-82 to 1996-97, the 33-34-week rate rose 31%, and the 35-36-week rate rose 51%. Women with less education, teenagers, unmarried women, primiparas, and blacks were more likely to deliver preterm across all three preterm birth levels. However, the effect of these low socioeconomic status markers diminished over the study period. Additionally, the odds of preterm birth among blacks increased with earlier gestational ages. Women who had intensive prenatal care utilization as compared with less than adequate utilization were more likely to deliver preterm (35-36 weeks) in 1996-97 (odds ratio (OR) = 2.05) compared with 1981-82 (OR = 1.44). Smaller increases were noted for <33 and 33-34 weeks.
  
  
  CONCLUSIONS
  Obstetric factors appear to be playing a greater role in the rise of twin preterm births at 35-36 weeks gestation. Temporal sociodemographic changes do not explain the rise in the preterm rate. Changing clinical practices may be having unintended consequences on the public health goals of reducing preterm and low birthweight rates in the United States.
authors.affiliation: Maternal and Child Health Bureau, Health Resources and Services Administration, Rockville, Maryland 20857, USA. mkogan@hrsa.gov
authors.forename: Michael D
authors.initials: MD
authors.lastname: Kogan
authors.forename: Greg R
authors.initials: GR
authors.lastname: Alexander
authors.forename: Milton
authors.initials: M
authors.lastname: Kotelchuck
authors.forename: Marian F
authors.initials: MF
authors.lastname: MacDorman
authors.forename: Pierre
authors.initials: P
authors.lastname: Buekens
authors.forename: Emile
authors.initials: E
authors.lastname: Papiernik
mesh_terms: D000293:Adolescent; D000328:Adult; D001741:Black or African American / Q000706:statistics & numerical data; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D016015:Logistic Models; D008297:Male; D007752:Obstetric Labor, Premature / Q000453:epidemiology*; D009774:Obstetrics / Q000706:statistics & numerical data; D011247:Pregnancy; D012307:Risk Factors; D012959:Socioeconomic Factors; D014427:Twins / Q000706:statistics & numerical data*; D014481:United States / Q000453:epidemiology