title: Reproductive factors and the risk of breast cancer in old age: a Norwegian cohort study.
abstract: Reproductive factors, including early age at menarche, late age at first birth, low parity, and late age at menopause, are associated with increased risk of breast cancer, but it is not clear to which degree the associations persist into old age. Altogether 58,426 Norwegian women born between 1886 and 1928 were followed up for breast cancer incidence from 1961 to 2008. Associations of reproductive factors with breast cancer risk were analyzed separately for the age intervals 55-69 and 70 years and older, using Cox regression. The associations were of similar strength in the two age strata. At 70 years and later, hazard ratios (HR) for the following comparisons were found: late (≥17 years) versus early (<13 years) age at menarche [HR 0.79, 95 % confidence interval (CI): 0.62, 1.01, P for trend <0.001]; late (≥35 years) versus early (<20 years) age at first birth (HR 1.54, 95 % CI: 1.13, 2.11, P for trend <0.001); high (≥5) versus low (1) parity (HR 0.68, 95 % CI: 0.54, 0.86, P for trend = 0.001) and late (50-54 years) versus early (<45 years) age at menopause (HR 1.44, 95 % CI: 1.10, 1.90, P for trend = 0.002). These findings suggest that reproductive events may have life-long effects on breast cancer risk.
authors.affiliation: Department of Public Health, Norwegian University of Science and Technology, Trondheim, Norway. julie.horn@ntnu.no
authors.forename: Julie
authors.initials: J
authors.lastname: Horn
authors.forename: Bjørn Olav
authors.initials: BO
authors.lastname: Åsvold
authors.forename: Signe
authors.initials: S
authors.lastname: Opdahl
authors.forename: Steinar
authors.initials: S
authors.lastname: Tretli
authors.forename: Lars J
authors.initials: LJ
authors.lastname: Vatten
mesh_terms: D017677:Age Distribution; D000368:Aged; D001943:Breast Neoplasms / Q000453:epidemiology*; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D016016:Proportional Hazards Models; D017584:Reproductive History; D012307:Risk Factors