title: Birth order and risk of childhood cancer in the Danish birth cohort of 1973-2010.
abstract: OBJECTIVE
  Many studies have investigated the possible association between birth order and risk of childhood cancer, although the evidence to date has been inconsistent. Birth order has been used as a marker for various in utero or childhood exposures and is relatively straightforward to assess.
  
  
  METHODS
  Data were obtained on all children born in Denmark between 1973 and 2010, involving almost 2.5 million births and about 5,700 newly diagnosed childhood cancers before the age of 20 years. Data were analyzed using Poisson regression models.
  
  
  RESULTS
  We failed to observe associations between birth order and risk of any childhood cancer subtype, including acute lymphoblastic leukemia; all rate ratios were close to one. Further analyses stratified by birth cohort (those born between 1973 and 1990, and those born between 1991 and 2010) also failed to show any associations. Considering stillbirths and/or controlling for birth weight and parental age in the analyses had no effect on the results. Analyses by years of birth (those born between 1973 and 1990, and those born between 1991 and 2010) did not show any changes in the overall pattern of no association.
  
  
  CONCLUSIONS
  In this large cohort of all children born in Denmark over an almost 40-year period, we did not observe an association between birth order and the risk of childhood cancer.
authors.affiliation: Section of Environment and Radiation, International Agency for Research on Cancer (IARC), 150 Cours Albert Thomas, 69372, Lyon, France. schuzj@iarc.fr.
authors.forename: Joachim
authors.initials: J
authors.lastname: Schüz
authors.affiliation: Department of Biostatistics, Bioinformatics and Biomathematics, Georgetown University, Washington, DC, USA.
authors.forename: George
authors.initials: G
authors.lastname: Luta
authors.affiliation: Section of Environment and Radiation, International Agency for Research on Cancer (IARC), 150 Cours Albert Thomas, 69372, Lyon, France.
authors.forename: Friederike
authors.initials: F
authors.lastname: Erdmann
authors.affiliation: Section of Environment and Radiation, International Agency for Research on Cancer (IARC), 150 Cours Albert Thomas, 69372, Lyon, France.
authors.forename: Gilles
authors.initials: G
authors.lastname: Ferro
authors.affiliation: Survivorship Unit, Childhood Cancer Survivorship Research Group, Danish Cancer Society Research Center, Copenhagen, Denmark.
authors.forename: Andrea
authors.initials: A
authors.lastname: Bautz
authors.affiliation: Survivorship Unit, Social Inequality in Survivorship Group, Danish Cancer Society Research Center, Copenhagen, Denmark.
authors.forename: Sofie Bay
authors.initials: SB
authors.lastname: Simony
authors.affiliation: Survivorship Unit, Social Inequality in Survivorship Group, Danish Cancer Society Research Center, Copenhagen, Denmark.
authors.forename: Susanne Oksbjerg
authors.initials: SO
authors.lastname: Dalton
authors.affiliation: Section of Environment and Radiation, International Agency for Research on Cancer (IARC), 150 Cours Albert Thomas, 69372, Lyon, France.|Epidemiology and Cancer Statistics Group, Department of Health Sciences, University of York, York, UK.
authors.forename: Tracy
authors.initials: T
authors.lastname: Lightfoot
authors.affiliation: Survivorship Unit, Childhood Cancer Survivorship Research Group, Danish Cancer Society Research Center, Copenhagen, Denmark.
authors.forename: Jeanette Falck
authors.initials: JF
authors.lastname: Winther
mesh_terms: D000293:Adolescent; D001722:Birth Order; D001724:Birth Weight / Q000502:physiology*; D002648:Child; D002675:Child, Preschool; D003718:Denmark; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D008297:Male; D009369:Neoplasms / Q000453:epidemiology* / Q000209:etiology; D012307:Risk Factors; D055815:Young Adult
keywords: Birth order; Childhood cancer; Denmark; Leukemia; Risk factors