A study found that women who breastfed for more than 6 months and later developed breast cancer were more likely to have an aggressive form of breast cancer. But it's important to know that the risk of developing breast cancer wasn't affected by how long a woman breastfed. These results were presented at the 2010 European Breast Cancer Conference.
Other studies have suggested that breastfeeding lowers breast cancer risk, but this link hasn't been found consistently in all studies. This study looked at two different questions:
- Can the length of time a woman breastfeeds affect her risk of developing breast cancer?
- Can the length of time a woman breastfeeds affect the aggressiveness of breast cancer if it does develop?
The researchers looked at the health histories of more than 14,000 women over 10 years. The women were split into groups depending on how long they had breastfed.
During the study, 424 women developed breast cancer. Each cancer was classified based on features that can make a cancer more aggressive, including cancer grade and Ki-67 values. Grade 3 cancers tend to be more aggressive than grade 1 or grade 2 cancers. Ki-67 is a marker associated with cancers that grow and spread more aggressively.
- There was no link between how long a woman breastfed and the risk of developing breast cancer.
- Women diagnosed with breast cancer who breastfed the longest time (more than 6 months) were about twice as likely as women diagnosed with breast cancer who breastfed the shortest time (less than 2 months) to develop breast cancer classified as aggressive (grade 3 and/or high levels of Ki-67).
The researchers don't know why there seems to be an association between breastfeeding for longer time periods and the likelihood of more aggressive breast cancer if disease develops.
The results DON'T mean that breast feeding for longer time periods is risky. Much more research needs to be done to confirm that this suggested association exists and if it's directly related to breastfeeding or to some other unknown but related factor. Results like these help doctors design future studies to pinpoint the causes of breast cancer.