Effects of birth order and maternal age on breast cancer risk: modification by whether women had been breast-fed.

BACKGROUND: Early life risk factors for breast cancer have been investigated in relation to hormonal, nutritional, infectious, and genetic hypotheses. Recent studies have also considered potential health effects associated with exposure to environmental contaminants in breastmilk.
METHODS: We analyzed data from a population-based case-control study of women living in Wisconsin. Cases (n = 2016) had an incident diagnosis of invasive breast cancer in 2002-2006 reported to the statewide tumor registry. Controls (n = 1960) of similar ages were randomly selected from driver's license lists. Risk-factor information was collected during structured telephone interviews. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated from multivariable logistic regression.
RESULTS: In multivariable models, maternal age and birth order were not associated with breast cancer risk in the full study population. The odds ratio for breast cancer risk associated with having been breast-fed in infancy was 0.83 (95% CI = 0.72-0.96). In analyses restricted to breast-fed women, maternal age associations with breast cancer were null (P = 0.2). Increasing maternal age was negatively associated with breast cancer risk among women who were not breast-fed; the odds ratio for breast cancer associated with each 5-year increase in maternal age was 0.90 (0.82-1.00). Higher birth order was inversely associated with breast cancer risk among breast-fed women (for women with 3 or more older siblings compared with first-born women, OR = 0.58 [CI = 0.39-0.86]) but not among nonbreast-fed women (1.13 [0.81-1.57]).
CONCLUSION: These findings suggest that early life risk factor associations for breast cancer may differ according to breast-feeding status in infancy.