http://www.thelancet.com/journal/vol360/iss9328/full/llan.360.9328.original_research.21812.1

Breast cancer and breastfeeding: collaborative reanalysis of individual
data from 47 epidemiological studies in 30 countries, including 50 302
women with breast cancer and 96 973 women without the disease 

Collaborative Group on Hormonal Factors in Breast Cancer* 

Summary 

Background Although childbearing is known to protect against breast
cancer, whether or not breastfeeding contributes to this protective
effect is unclear. 

Methods Individual data from 47 epidemiological studies in 30 countries
that included information on breastfeeding patterns and other aspects of
childbearing were collected, checked, and analysed centrally, for 50 302
women with invasive breast cancer and 96 973 controls. Estimates of the
relative risk for breast cancer associated with breastfeeding in parous
women were obtained after stratification by fine divisions of age,
parity, and women's ages when their first child was born, as well as by
study and menopausal status. 

Findings Women with breast cancer had, on average, fewer births than did
controls (2·2 vs 2·6). Furthermore, fewer parous women with cancer than
parous controls had ever breastfed (71% vs 79%), and their average
lifetime duration of breastfeeding was shorter (9·8 vs 15·6 months). The
relative risk of breast cancer decreased by 4·3% (95% CI 2·9-5·8;
p<0·0001) for every 12 months of breastfeeding in addition to a decrease
of 7·0% (5·0-9·0; p<0·0001) for each birth. The size of the decline in
the relative risk of breast cancer associated with breastfeeding did not
differ significantly for women in developed and developing countries,
and did not vary significantly by age, menopausal status, ethnic origin,
the number of births a woman had, her age when her first child was born,
or any of nine other personal characteristics examined. It is estimated
that the cumulative incidence of breast cancer in developed countries
would be reduced by more than half, from 6·3 to 2·7 per 100 women by age
70, if women had the average number of births and lifetime duration of
breastfeeding that had been prevalent in developing countries until
recently. Breastfeeding could account for almost two-thirds of this
estimated reduction in breast cancer incidence. 

Interpretation The longer women breast feed the more they are protected
against breast cancer. The lack of or short lifetime duration of
breastfeeding typical of women in developed countries makes a major
contribution to the high incidence of breast cancer in these countries. 

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http://www.thelancet.com/journal/vol360/iss9328/full/llan.360.9328.original_research.21812.1

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