Birth intervals and breast cancer risk

BACKGROUND: The interval between successive births (birth interval) may affect breast cancer risk, whereas interval from last birth to cancer onset may modify its behaviour. METHODS: The study cohort consisted of 29 488 Finnish grand multiparous (GM) women, including 628 women with breast cancer. Conditional logistic regression for case–control design nested within the cohort was used to estimate proportional hazards (referred as relative risks, RR). Age at first birth and parity were co-variables. RESULTS: Short interval (<1 year) between first and second birth increased the risk of advanced ductal breast cancer at ages < 50 years (RR=5.29; 95% CI 2.00–14.0) as compared to interval 3+ years. The risk of advanced ductal cancer was also large (RR = 4.00; 95% CI 1.19–13.4) shortly (<3 years) after last birth as compared with the period 15+ years. CONCLUSIONS: Short birth interval-associated excess breast cancer risk may be related to stimulatory effects of female steroid hormones produced during two closely connected pregnancies, or defective breast maturation owing to failures in breastfeeding.