Research has shown that childbearing seems to reduce a woman’s risk of developing breast cancer. However, it has been difficult to determine if breast-feeding also plays a role in the prevention of the disease. Research in the July 20, 2002 issue of The Lancet not only suggests that a woman who breast feeds reduces her risk of breast cancer, but it also suggests that the longer she breast-feeds the lower the risk.

About the study

An international group of researchers, called the Collaborative Group on Hormonal Factors in Breast Cancer, analyzed data from 47 studies conducted in 30 countries. In total, the studies included 50,302 women with invasive breast cancer and 96,973 women without the disease.

Studies chosen for this re-analysis had to be of a certain and size (100 or more women with invasive breast cancer) and have collected information on breast-feeding, other aspects of childbearing, and use of hormone therapies.

Researchers looked very closely at the following data from each study:

  • Whether or not a woman gave birth to any children
  • Age when first child was born
  • Number of births for each woman
  • Whether or not a woman breast-fed
  • How many children a woman breast-fed
  • How many months a woman breast-fed each child
  • Whether or not each woman developed breast cancer

Because studies are designed differently from one another, the researchers used sophisticated statistical methods to analyze the data from all of the studies together. In essence, they compared the risk of breast cancer among all women who had breast-fed with the risk among all women who had not. They also investigated how the duration of breast-feeding influenced this risk.

The findings

Among women who had given birth, those who breast-fed were less likely to develop breast cancer than those who did not. In addition, those with longer lifetime duration of breast-feeding were less likely to develop breast cancer. Specifically, for every year a woman breast-fed, her risk of developing breast cancer was reduced by 4.3%.

As expected, women who gave birth to more children were also less likely to develop breast cancer than women who gave birth to fewer children. The risk of breast cancer declined by 7% for every child a woman birthed.

In calculating these statistics, the researchers controlled for other factors that affect the risk of breast cancer, such as age, menopausal status, ethnicity, age at first birth, and other reproductive factors.

Although these results seem to further support the many benefits of breast-feeding, this study has its limitations. First, it is a meta-analysis, meaning that the researchers pooled and analyzed data from many studies in an attempt to derive an overall estimate of risk or benefit. Because studies are designed differently, meta-analyses lack a certain degree of precision, though they are useful in drawing general conclusions from many similar studies. Second, the findings are based on women’s estimates of how long they breast-fed, which may not be completely accurate. In fact, the women tended to round their estimates to multiples of 6 or 12 months, which is fairly imprecise. Third, there was no distinction between exclusive breast-feeding and breast-feeding that was supplemented with formula—a factor that could conceivably make a difference Fourth, relatively few women in these studies breast-fed for longer than 30 months (lifetime total), which diminishes the strength of the risk/benefit estimates associated with longer-term breast-feeding. Finally, it is still possible that the breast-feeding itself did not lower the risk, but another unidentified factor (or factors) associated with both breast-feeding and breast cancer risk was responsible.

How does this affect you?

Will you get breast cancer if you don’t breast-feed? Chances are you won’t. Most women who do not breast feed don’t get breast cancer. Currently, there is no way to know definitively who will develop breast cancer. However, the findings from this analysis suggest that, in addition to childbearing, breast-feeding may also help reduce a woman’s risk of developing breast cancer.

Although breast-feeding may be one factor that reduces a woman’s risk of breast cancer, it is certainly not the only factor determining her risk. Research shows that a number of factors work together to increase or decrease breast cancer risk—including age, genes, childbearing history, menstrual history, and use of hormone therapies.

Aside from a potential reduction in breast cancer risk, there are plenty of reasons to breast-feed. Breast milk is free, and research shows that women who breast-feed return to pre-pregnancy weight more quickly and have less postpartum bleeding than women who don’t. And breast-feeding provides numerous benefits for baby, as well. According to the American Academy of Pediatrics, breast milk contains all the protein, sugar, fat, and vitamins your baby needs, and it also helps protect your baby from infections and other diseases.