Aug. 25, 2008 -- Certain types of breast cancer may be rarer among women who breastfeed their babies for at least six months.
That finding comes from a new study published in today's advance online edition of Cancer.
The researchers, who included Amanda Phipps, MPH, of the Fred Hutchinson Cancer Research Center in Seattle, pooled data from two breast cancer studies that together included nearly 2,500 women aged 55-79 in Washington state. The group included 1,140 women who had had breast cancer.
Phipps and colleagues combed through the data looking for patterns that stood out among women with any of the following types of breast cancer:
- Estrogen-sensitive breast cancer (the majority of breast cancers)
- HER2-positive breast cancer (breast cancers with high levels of the HER2 protein)
- "Triple negative" breast cancer (breast cancers not sensitive to estrogen or progesterone and not HER2-positive)
Breastfeeding for at least six months appeared to be most protective for "triple negative" breast cancer. Triple-negative breast cancer was half as common among women who reported breastfeeding their babies for six or more months than among mothers who hadn't breastfed.
By the same comparison, estrogen-sensitive breast cancer was 20% less common among women who breastfed for at least six months than among mothers who didn't breastfeed.
The reason for those findings isn't clear. The study doesn't prove that breastfeeding prevented breast cancer or that not breastfeeding raises breast cancer risk; observational studies, such as this one, don't prove cause and effect. But other observational research has linked breastfeeding to lower rates of breast cancer, Phipps' team notes.
Besides breastfeeding, two other patterns emerged:
- Early menarche -- starting menstruation at or before age 13 -- was only linked to increased risk of HER2-positive breast cancer.
- Late menopause -- after age 55 -- and use of estrogen-plus-progesterone hormone therapy were only linked to risk of estrogen-sensitive breast cancer
Phipps and colleagues considered other factors, including the women's education level, smoking status, alcohol consumption, and immediate family history of breast cancer. Still, they caution that relatively few women in the studies had HER2-positive or triple-negative breast cancer, which may have made it harder to spot trends in those types of cancer. Phipps' team concludes that "certain reproductive factors may have a greater impact" than others on the risk of developing certain subtypes of breast cancer and that further research is needed.