ArticlesBreast 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*
Introduction
Although childbearing is known to protect against breast cancer, what contribution breastfeeding has on this protective effect, if any, has been difficult to determine. Breastfeeding is closely related to many other aspects of childbearing—for example, women breastfeed only after they have had a child, and the earlier they commence childbearing, the more children they have and the longer their lifetime duration of breastfeeding. No single study has been large enough to reliably characterise the relative contributions of such closely related factors in breast cancer. This study combines data from 47 epidemiological studies conducted in 30 countries, to examine the relation between breastfeeding and breast cancer, taking careful account of the effects of other related aspects of childbearing.
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Contributing studies and collection of data
The Collaborative Group on Hormonal Factors in Breast Cancer has brought together worldwide data from epidemiological studies of women with breast cancer to describe the relation between breast cancer and various reproductive, hormonal, and other factors.1, 2, 3, 4 Case control and cohort studies were eligible for the collaboration if they had data for at least 100 women with incident invasive breast cancer and had recorded information on each woman with respect to reproductive factors and use
Results
Altogether 50 302 women with invasive breast cancer (cases) and 96 973 women without breast cancer (controls) from 47 studies in 30 countries are included in these analyses (figure 1). Among the cases, the median year of diagnosis was 1988 and the average age at diagnosis was 50·1 years. Cases had, on average, fewer births than did controls (2·2 vs 2·6) and a greater proportion were nulliparous (16% vs 14%). The proportion of parous women who had ever breastfed was also lower in cases than in
Discussion
Our analyses here show that the relative risk of breast cancer is reduced by 4·3% (95% CI 2·9–5·8) for each year that a woman breastfeeds, in addition to a reduction of 7·0% (5·0–9·0) for each birth. These relations are significant and are seen consistently for women from developed and developing countries, of different ages and ethnic origins, and with various childbearing patterns and other personal characteristics.
The 47 studies that contributed data were of different designs and included
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