title: Immortal time bias in the study of stillbirth risk factors: the example of gestational diabetes.
abstract: BACKGROUND
  Current understanding of the increased risk for stillbirth in gestational diabetes mellitus is often based on large cohort studies in which the risk of stillbirth in women with this disease is compared with the risk in women without. However, such studies could be susceptible to immortal time bias because, although many cohorts begin at 20 weeks' gestation, pregnancies must "survive" until 24-28 weeks in order to be screened and diagnosed with gestational diabetes.
  
  
  METHODS
  We describe the theoretical potential for immortal time bias in studies of stillbirth and gestational diabetes and then quantify the magnitude of the bias using 2006 United States vital statistics data.
  
  
  RESULTS
  Although gestational diabetes was protective against stillbirth when including all births (relative risk = 0.88 [95% confidence interval = 0.79-0.99]), restricting analyses to births at >28 weeks' gestation reversed the effect and diabetes became associated with an increased risk of stillbirth (1.25 [1.11-1.41]).
  
  
  CONCLUSIONS
  Immortal time before diagnosis of gestational diabetes may bias our understanding of the stillbirth risk associated with this condition.
authors.affiliation: From the aUniversity of British Columbia, Department of Obstetrics & Gynaecology, Vancouver, Canada; and bUniversity of British Columbia, School of Population and Public Health, Vancouver, Canada.
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Hutcheon
authors.forename: Verena
authors.initials: V
authors.lastname: Kuret
authors.forename: K S
authors.initials: KS
authors.lastname: Joseph
authors.forename: Yasser
authors.initials: Y
authors.lastname: Sabr
authors.forename: Kenneth
authors.initials: K
authors.lastname: Lim
mesh_terms: D015982:Bias; D015331:Cohort Studies; D016640:Diabetes, Gestational / Q000175:diagnosis* / Q000453:epidemiology; D005260:Female; D005865:Gestational Age; D006801:Humans; D011247:Pregnancy; D012307:Risk Factors; D050497:Stillbirth / Q000453:epidemiology*; D013997:Time Factors; D014481:United States / Q000453:epidemiology