Elsevier

The Lancet

Volume 360, Issue 9328, 20 July 2002, Pages 187-195
The Lancet

Articles
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*

https://doi.org/10.1016/S0140-6736(02)09454-0Get rights and content

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.

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.

Section snippets

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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