Background: Breast cancer incidence continuous to increase. We examined at population level the association between the relative excess risk of breast cancer and previous age of mother at first birth.
Method: Incidence of breast cancer in 34 industrialized countries was obtained from the GLOBOCAN 2002 and SEER databases. Data on age of mother at first birth was collected through national statistics offices. National relative excess risk (RER) was calculated by subtracting the lowest age-specific incidence rate from the rate in each population, and dividing the difference by the latter.
Results: The national RER in 2002 correlated closely with a higher average age at first birth in 1972, 1982, 1992 and also 2002, Pearson correlation [r] being 0.83, 0.79, 0.72 and 0.61, respectively; P < 0.0001. RER of breast cancer in 2002 for those aged 15-44 years correlated closely with the mean age at first birth in 1982 and 1992 (r: 0.81 and 0.75; P < 0.0001), whereas RER for those aged 45-54 years correlated strongly with age at first birth in 1972 and 1982 (r: 0.81 and 0.76; P < 0.0001), and for those aged 55-64 years with age at first birth in 1972 (r: 0.77; P < 0.0001).
Conclusions: The rising age at first childbirth of mothers has been followed by marked increases in breast cancer incidence. Later age at first birth seems to characterize secular diffusion of 'modern' lifestyles with a potentially large impact on increased breast cancer risk, and hence should be accompanied by greater opportunities for prevention through modifiable risk factors.