title: The joint influence of marital status, interpregnancy interval, and neighborhood on small for gestational age birth: a retrospective cohort study.
abstract: BACKGROUND
  Interpregnancy interval (IPI), marital status, and neighborhood are independently associated with birth outcomes. The joint contribution of these exposures has not been evaluated. We tested for effect modification between IPI and marriage, controlling for neighborhood.
  
  
  METHODS
  We analyzed a cohort of 98,330 live births in Montréal, Canada from 1997-2001 to assess IPI and marital status in relation to small for gestational age (SGA) birth. Births were categorized as subsequent-born with short (<12 months), intermediate (12-35 months), or long (36+ months) IPI, or as firstborn. The data had a 2-level hierarchical structure, with births nested in 49 neighborhoods. We used multilevel logistic regression to obtain adjusted effect estimates.
  
  
  RESULTS
  Marital status modified the association between IPI and SGA birth. Being unmarried relative to married was associated with SGA birth for all IPI categories, particularly for subsequent births with short (odds ratio [OR] 1.60, 95% confidence interval [CI] 1.31-1.95) and intermediate (OR 1.48, 95% CI 1.26-1.74) IPIs. Subsequent births had a lower likelihood of SGA birth than firstborns. Intermediate IPIs were more protective for married (OR 0.50, 95% CI 0.47-0.54) than unmarried mothers (OR 0.65, 95% CI 0.56-0.76).
  
  
  CONCLUSIONS
  Being unmarried increases the likelihood of SGA birth as the IPI shortens, and the protective effect of intermediate IPIs is reduced in unmarried mothers. Marital status should be considered in recommending particular IPIs as an intervention to improve birth outcomes.
authors.affiliation: Unité Etudes et analyses de l'état de santé de la population, Institut national de santé publique du Québec, Montréal, Québec, Canada. nathalie.auger@inspq.qc.ca
authors.forename: Nathalie
authors.initials: N
authors.lastname: Auger
authors.forename: Mark
authors.initials: M
authors.lastname: Daniel
authors.forename: Robert W
authors.initials: RW
authors.lastname: Platt
authors.forename: Zhong-Cheng
authors.initials: ZC
authors.lastname: Luo
authors.forename: Yuquan
authors.initials: Y
authors.lastname: Wu
authors.forename: Robert
authors.initials: R
authors.lastname: Choinière
mesh_terms: D000328:Adult; D001721:Birth Intervals / Q000706:statistics & numerical data*; D015331:Cohort Studies; D016001:Confidence Intervals; D005260:Female; D006304:Health Status; D006801:Humans; D007231:Infant, Newborn; D007236:Infant, Small for Gestational Age; D016015:Logistic Models; D008297:Male; D017533:Marital Status / Q000706:statistics & numerical data*; D008425:Maternal Behavior; D016017:Odds Ratio; D011203:Poverty / Q000706:statistics & numerical data; D011247:Pregnancy; D011256:Pregnancy Outcome / Q000453:epidemiology*; D011792:Quebec / Q000453:epidemiology; D012189:Retrospective Studies; D012307:Risk Factors; D012931:Social Environment; D012959:Socioeconomic Factors