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And I'm Renee Montagne.

Today in Your Health, women and breast cancer. In a few moments, one mother calculates her risk of getting the disease. But first, there's new research on the protective effects of breastfeeding.

Here's NPR's Richard Knox.

RICHARD KNOX: It's another birthday at New England's busiest maternity ward.

(Soundbite of baby crying)

KNOX: On the ninth floor of Brigham and Women's Hospital, a half-dozen brand new moms and dads, looking dazed, file into a breastfeeding class.

Ms. GINA ABBASCIA-SIMMONS (Lactation Consultant): So thank you for coming. My name in Gina Abbascia. Congratulations for making the healthiest choice to breastfeed your baby or give breast milk.

KNOX: More new mothers are breastfeeding these days - more than 80 percent at this hospital. Most women say they do it because of the benefits for their babies.

For instance, breastfed babies get a shot of their mother's antibodies before their own immune systems kick in. That means fewer infections now, and maybe less asthma later on. But there's growing evidence that mothers benefit too, in having a lower risk of cancer.

Doctors have known for a while that women who first give birth in their early 20s have lower rates of breast cancer later in life than women who delay childbearing. This made Dr. Geske Ursin and her colleagues wonder if the hormonal changes associated with pregnancy and breastfeeding were somehow protective.

Dr. GESKE URSIN (Researcher, University of Southern California): We had looked at giving birth early, giving birth late, as well as breastfeeding, and we did see a very impressive and protective effects of breastfeeding. And we wanted to further understand how it works.

KNOX: Ursin, who's at the University of Southern California, was part of a large national study that collected data on 9,000 women over a period of years. They found that breastfeeding cancels out the effect of delaying childbirth past the age of 25 - the average age of first birth these days.

Dr. URSIN: The good news is that breastfeeding protects against breast cancer no matter when you give birth - whether you give birth before age 25 or after age 25.

KNOX: And earlier this week, Ursin reported further good news at a meeting of the American Association for Cancer Research in Los Angeles. Breastfeeding lowers the risk of later breast cancer of the worst type - a kind whose growth is not promoted by hormones. That was unexpected. Women who give birth after age 25 and have three or more children have double the risk of these aggressive and hard to treat breast cancers.

Dr. URSIN: This two-fold increased risk went away when we only looked at women who breastfed. They were no longer at increased risk of breast cancer.

KNOX: The mechanism is unclear. Some experts think it's because a woman is less likely to ovulate while she's breastfeeding. And the fewer times a woman ovulates, the lower her risk of breast cancer.

Whatever the reason, the bottom line is clear: breastfeeding offers broad protection against both major types of breast cancer in all women. At the Brigham and Women's Hospital, Susan Bradley has decided to breastfeed Michaela Rose, born yesterday morning.

Ms. SUSAN BRADLEY (Mother): I'm going to try. So, I mean, we just started and already, you know, it's definitely a rush. It's a good experience so far.

KNOX: To Bradley, the idea that she might be lowering her risk of breast cancer is a definite bonus.

Ms. BRADLEY: You hear about all the other advantages to the baby, but that's good to know. It's kind of more motivating, too, to try to stick with it.

KNOX: About 70 percent of Caucasian women like Bradley now breastfeed their babies. The rate is much lower among African-American women. Ursin says that could mean more breast cancers for them later on.

Richard Knox, NPR News, Boston.

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