Reproductive factors and the risk of breast cancer in old age: a Norwegian cohort study

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 (a parts per thousand yen17 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 (a parts per thousand yen35 years) versus early (< 20 years) age at first birth (HR 1.54, 95 % CI: 1.13, 2.11, P for trend < 0.001); high (a parts per thousand yen5) 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.