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Birth intervals and breast cancer risk
A Kauppila1, P Kyyrönen, M Hinkula
1Department of Obstetrics and Gynecology, Oulu University Hospital, FI-90250 Oulu, Finland.
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.
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