"Women with two or more kids had 1.5 times the risk of getting ER-negative breast cancer compared with women who had no kids," says Palmer, a professor of epidemiology. "We found that increased risk was really only there for women who had not breastfed."
Palmer's study is published in Cancer Epidemiology, Biomarkers & Prevention.
ER-Negative Breast Cancer
"ER-negative is more difficult to treat," Palmer says. It has a worse outlook.
They evaluated women who participated in the Black Women's Health Study. In this ongoing study, 59,000 African-American women were followed from 1995 until 2009.
When they first entered the study, the women answered detailed questions about childbirth and breastfeeding.
They completed another questionnaire every two years. They told researchers whether they had any diagnosis of cancer.
For this study, Palmer identified 457 women with ER-positive cancer and 318 with ER-negative.
''For the most common type of cancer, ER-positive, we found what everyone has found for breast cancer overall," she says. "The more babies you have, the lower the risk of developing [this] cancer."
Breastfeeding didn't affect that association.
"But for the other subtype, we found the opposite relationship," she says. "The more babies you have, the higher the risk of [ER-negative] breast cancer you have."
However, the risk declined in those who had breastfed.
If a woman does not breastfeed, the breast goes through a process called involution, Palmer says. This is marked by increasing inflammation in the breast tissue. This inflammation may be associated with breast cancer.
Women who breastfeed typically reduce the number of breastfeeding sessions, stopping gradually, she says. As result, experts believe the breast returns to its pre-pregnancy state in a more orderly manner, and with less inflammation.
African-American Moms Less Likely to Breastfeed
While previous research has found similar links between breastfeeding and reduced breast cancer risk, the new study is strong because it followed people over time, says Alison Stuebe, MD, assistant professor of obstetrics and gynecology, University of North Carolina School of Medicine. She was not involved in the study.
The new study provides another strong argument for promoting breastfeeding, she says. "African-Americans are much less likely to initiate breastfeeding," she says, compared to other ethnic groups.
This study and others suggest that the healthful effects of breastfeeding extend to the mothers as well, she says. "Breastfeeding is good for mothers and it's good for babies."
Doctors need to talk more about the value of breastfeeding, she says.
However, obstacles are plentiful. She cites a lack of advice about breastfeeding to new mothers right after they give birth. She tells mothers to have the baby ''skin to skin'' with them as soon as possible after birth. That way, the baby is likely to latch on and begin to breastfeed successfully.
Before birth, a woman should find a breastfeeding support team, Stuebe says. "Find a friend or relative who has experienced breastfeeding who can be a go-to."
Women can also ask their pediatrician for a referral to a lactation consultant.