title: Gestational age-dependent risk factors for preterm birth: associations with maternal education and age early in gestation.
abstract: OBJECTIVE
  Preterm birth (PTB) before 37 weeks can occur over a wide range of gestational ages, but few studies have assessed if associations between risk factors and PTB vary over the duration of gestation. We sought to evaluate if associations between two major risk factors (maternal education and age) and PTB depend on gestational age at delivery.
  
  
  METHODS
  We estimated hazard ratios of PTB for education and age in a time-to-event analysis using a retrospective cohort of 223,756 live singleton births from the province of Québec, Canada for the years 2001-2005. Differences in hazards of maternal education and age with PTB were assessed over gestational age in a Cox proportional hazards model using linear and nonlinear time interaction terms, adjusting for maternal characteristics.
  
  
  RESULTS
  Associations of PTB with lower (vs. higher) education and older (vs. younger) age strengthened progressively at earlier gestational ages, such that the risk of PTB for maternal education and age was not constant over the course of gestation.
  
  
  CONCLUSIONS
  Associations of PTB with risk factors such as maternal low education and older age may be stronger early in gestation. Models that capture the time-dependent nature of PTB may be useful when the goal is to assess associations at low gestational ages, and to avoid masked or biased associations early in gestation.
authors.affiliation: Institut national de santé publique du Québec, Montréal, Canada; Research Centre of the University of Montréal Hospital Centre, Montréal, Canada; Department of Social and Preventive Medicine, University of Montréal, Montréal, Canada. Electronic address: nathalie.auger@inspq.qc.ca.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Auger
authors.affiliation: Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montréal, Canada.
authors.forename: Michal
authors.initials: M
authors.lastname: Abrahamowicz
authors.affiliation: Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montréal, Canada.
authors.forename: Willy
authors.initials: W
authors.lastname: Wynant
authors.affiliation: Institut national de santé publique du Québec, Montréal, Canada; Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montréal, Canada.
authors.forename: Ernest
authors.initials: E
authors.lastname: Lo
mesh_terms: D000328:Adult; D004522:Educational Status; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D008423:Maternal Age; D011247:Pregnancy; D047928:Premature Birth / Q000209:etiology*; D016016:Proportional Hazards Models; D012189:Retrospective Studies; D012306:Risk
keywords: Educational status; Gestational age; Maternal age; Preterm birth; Survival analysis