For every 12 months that a woman breastfeeds, her risk of breast cancer declines by 4%, according to an analysis of 47 epidemiologic studies in 30 countries; this reduction is essentially the same in developing and developed countries, and for women with different background characteristics and reproductive histories.1 In addition, breast cancer risk is reduced by 7% for every birth a woman has. The incidence of breast cancer is much lower in developing than in developed countries, and findings from this analysis suggest that the larger families and patterns of prolonged breastfeeding typical in the developing world explain much of the difference.

To study the relationship between reproductive factors and breast cancer risk, the analysts combined data from cohort and case-control studies involving a total of more than 50,000 women with breast cancer and nearly 97,000 cancer-free controls. Women with breast cancer were, on average, 50 years old when they received their diagnosis. These women had had fewer births than controls (2.2 vs. 2.6), and a larger proportion of them had never given birth (16% vs. 14%).

To calculate the relative risks of breast cancer associated with breastfeeding, the analysts limited the sample to women who had given birth, and they stratified the data by study, specific study site, and women's parity, age at diagnosis, age at first birth and menopausal status. Among women who had given birth, those with cancer were less likely than controls to have breastfed (71% vs. 79%) and reported a shorter average lifetime duration of breastfeeding (9.8 vs. 15.6 months).

For both cases and controls, as lifetime duration of breastfeeding increased, mean parity and mean number of children breastfed increased, and age at first birth decreased. To assess the risk of breast cancer associated with reproductive factors in the absence of breastfeeding, the analysts examined data on women who had never breastfed; among this group, they found that the relative risk of breast cancer declined by 3% for each one-year decrease in a woman's age at first birth and by 7% for each birth she had. When they compared the effects of parity for women who had breastfed and those who had not, the risk of breast cancer declined as the number of births increased for both groups, but at each parity, women who had never breastfed had a slightly higher relative risk than those who had ever breastfed.

Comparisons of women who had never breastfed with those who had breastfed for varying durations revealed that the relative risk of breast cancer was 0.9 for women who had breastfed for 7-18 months and fell to 0.7 for those who had breastfed for 55 months or more. The reduction in relative risk was 4% for every 12 months of breastfeeding over a woman's lifetime, and this result was the same irrespective of a woman's parity, age at first birth or at diagnosis, ethnicity, level of education, family history of breast cancer, age at menarche, height, weight, body mass index, history of hormonal contraceptive use, menopausal status, or alcohol or tobacco use. The same reduction also was seen for both women in the developing world and their counterparts in developed countries.

Citing findings from other research, the analysts observe that around 1990, women living in developed countries had a considerably higher cumulative incidence of breast cancer until age 70 than those in developing countries. Given that women in the developed world also had fewer births and breastfed for shorter durations, the analysts explored the contribution of these factors to the incidence of breast cancer. By applying the relative risks from their study to age-specific incidence rates typical for developed countries around 1990, they estimated that if each woman in the developed world had 6.5 births instead of 2.5 and breastfed each child for 24 months rather than three months, the incidence of breast cancer by age 70 would be reduced by more than half (from 6.3 to 2.7 cases per 100 women). They further estimated that breastfeeding would be responsible for almost two-thirds of this reduction.

As the analysts acknowledge, it is "unrealistic" to expect that the incidence of breast cancer will be substantially lowered by women's adopting patterns of childbearing and breastfeeding that were typical in much of the world until about a century ago. However, they suggest that understanding how breastfeeding protects against breast cancer may make it possible to prevent the disease "by mimicking the effect of breastfeeding therapeutically or in some other way." They conclude that "in the meantime, important reductions in breast-cancer incidence could be achieved if women considered breastfeeding each child for longer than they do now."--D. Hollander


1. Collaborative Group on Hormonal Factors in Breast Cancer, 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, Lancet, 2002, 360(9328):187-195.