title: Influence of family size and birth order on risk of cancer: a population-based study.
abstract: BACKGROUND
  Family size and birth order are known to influence the risk of some cancers. However, it is still unknown whether these effects change from early to later adulthood. We used the data of the Swedish Family-Cancer Database to further analyze these effects.
  
  
  METHODS
  We selected over 5.7 million offspring with identified parents but no parental cancer. We estimated the effect of birth order and family size by Poisson regression adjusted for age, sex, period, region and socioeconomic status. We divided the age at diagnosis in two groups, below and over 50 years, to identify the effect of family size and birth order for different age periods.
  
  
  RESULTS
  Negative associations for increasing birth order were found for endometrial, testicular, skin, thyroid and connective tissue cancers and melanoma. In contrast, we observed positive association between birth order and lung, male and female genital cancers. Family size was associated with decreasing risk for endometrial and testicular cancers, melanoma and squamous cell carcinoma; risk was increased for leukemia and nervous system cancer. The effect of birth order decreased for lung and endometrial cancer from age at diagnosis below to over 50 years. Combined effects for birth order and family size were marginally significant for thyroid gland tumors. Especially, the relative risk for follicular thyroid gland tumors was significantly decreased for increasing birth order.
  
  
  CONCLUSIONS
  Our findings suggest that the effect of birth order decreases from early to late adulthood for lung and endometrial cancer.
authors.affiliation: Division of Molecular Genetic Epidemiology, German Cancer Research Center, Im Neuenheimer Feld 580, D-69120 Heidelberg, Germany. m.bevier@dkfz-heidelberg.de
authors.forename: Melanie
authors.initials: M
authors.lastname: Bevier
authors.forename: Marianne
authors.initials: M
authors.lastname: Weires
authors.forename: Hauke
authors.initials: H
authors.lastname: Thomsen
authors.forename: Jan
authors.initials: J
authors.lastname: Sundquist
authors.forename: Kari
authors.initials: K
authors.lastname: Hemminki
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D001722:Birth Order; D002648:Child; D002675:Child, Preschool; D016208:Databases, Factual; D005191:Family Characteristics; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008875:Middle Aged; D009369:Neoplasms / Q000453:epidemiology*; D012042:Registries; D012307:Risk Factors; D013548:Sweden / Q000453:epidemiology; D055815:Young Adult