title: Birth weight-breast cancer revisited: is the association confounded by familial factors?
abstract: OBJECTIVE
  The study aimed to investigate whether the association between birth weight and the risk of breast cancer can be confounded by familial factors, such as shared environment and common genes.
  
  
  METHODS
  Eligible were all female like-sexed twins of the Swedish Twin Registry, born during the period 1926-1958 and alive in 1973. Data were obtained from birth records, and the final study population with reliable birth weight data was made up of 11,923 twins. Hazard ratios (HR) for breast cancer according to birth weight were estimated through Cox regression, using robust SE to account for the dependence within twin pairs. Paired analysis was done to account for potential confounding by familial factors.
  
  
  RESULTS
  In the cohort analysis, a birth weight >or=3,000 g was associated with an increased risk of breast cancer diagnosed at or before 50 years [adjusted HR, 1.57; 95% confidence interval (95% CI), 1.03-2.42] but not with breast cancer with a later onset (adjusted HR, 0.80; 95% CI, 0.57-1.12). From >or=2,500 g, a 500-g increase in birth weight conferred a HR of 1.62 (95% CI, 1.16-2.27) for breast cancer diagnosed at or before 50 years. This risk remained in analysis within twin pairs (HR, 1.57; 95% CI, 1.00-2.48).
  
  
  CONCLUSIONS
  In the present study, findings indicate that the association between birth weight and breast cancer risk, seen only in women diagnosed early (<or=50 years), is not confounded by familial factors.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Box 281 Karolinska Institutet, 17177 Stockholm, Sweden. sara.oberg@ki.se
authors.forename: Sara
authors.initials: S
authors.lastname: Oberg
authors.forename: Sven
authors.initials: S
authors.lastname: Cnattingius
authors.forename: Sven
authors.initials: S
authors.lastname: Sandin
authors.forename: Paul
authors.initials: P
authors.lastname: Lichtenstein
authors.forename: Anastasia
authors.initials: A
authors.lastname: Iliadou
mesh_terms: D001724:Birth Weight; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000235:genetics; D015331:Cohort Studies; D004200:Diseases in Twins / Q000453:epidemiology* / Q000235:genetics; D004777:Environment; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D011247:Pregnancy; D016016:Proportional Hazards Models; D012307:Risk Factors; D013548:Sweden / Q000453:epidemiology