breast feeding and breast cancer risk
Last reviewed 01/2018
- a meta-analysis has been undertaken by the Collaborative Group on Hormonal
Factors in Breast Cancer investigating the risk of breast cancer associated
with breast feeding. In the meta-analysis the results of 42 case control studies
were combined with those of 5 cohort studies (included by using a nested case
control design) to obtain odds ratios - note that the incidence of breast
cancer cannot be computed from case control studies and so the calculation
of relative risk was approximated from odds ratios
- in all parous women, the relative risk (RR) of breast cancer decreased with increasing duration of breast feeding (reduction in the RR per 12 months of breastfeeding 4.3%, 95% CI 2.9% to 5.8%)
- in the absence of breast feeding each birth reduced the RR of breast cancer by 7.0% (CI 5% to 9%)
- the overall effect of breast feeding did not vary by country (developed vs developing), education, age at diagnosis, previous use of hormonal contraceptives, use of alcohol or tobacco, ethnic origin, menopausal status, body mass index, parity, age at menarche, and age at first birth of the women
- the study authors conclude that the relative risk of breast cancer decreases with increasing duration of breast feeding. The effect is consistent after adjustment of possible confounders
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