We all know that breastfeeding is good for the baby. It provides much-needed antibodies to lower a baby's risk of diarrhea, ear infections, and bacterial meningitis, as well as preventing overfeeding, which could stave off obesity later in life. But is it good for moms? That's a difficult question to answer. Some studies suggest it may help prevent bone loss later in life and possibly lower the risk of ovarian cancer by delaying the onset of menstruation after pregnancy. Now, an intriguing new study out today in the Archives of Internal Medicine shows that it could help some women avoid breast cancer early in life. Breastfeeding conferred nearly a 60 percent lower risk of premenopausal breast cancer on women who had a family history of the disease.

There are, though, a lot of caveats to this study. First of all, it shows a protective effect only against breast tumors that develop before menopause. Yet the vast majority of breast cancers occur well beyond a woman's 40s. What's more, the study of 60,000 nurses who had given birth found that the protective effect extended only to women already at elevated risk of breast cancer--namely, those who had a close relative (mother, sister, grandmother) who'd been diagnosed with the disease. Women with no family history of breast cancer didn't get any extra protection by breastfeeding.

Still, the study offers some interesting take-home messages. The researchers point out that the breast cancer preventive benefits of breastfeeding compare similarly to the protective effects of the antiestrogen drug tamoxifen, which many high-risk women take to prevent breast cancer. What's more, it didn't really matter how long a woman breastfed, whether it was one child who was nursed for two months, for instance, or three children nursed for three years. "These data suggest that women with a family history of breast cancer should be strongly encouraged to breastfeed," the authors write.

Certainly, women are already getting that message. Nearly three quarters of new moms breastfeed at some point during the first six months of their babies' lives. That compares with fewer than 20 percent of moms in the 1950s, many of whom were told by pediatricians that formula was just as good as breast milk. The American Academy of Pediatrics now strongly urges women to breastfeed exclusively for the first six months and continue breastfeeding, with supplemental food, for the first year or more.

The researchers were able to rule out body weight as a potential factor in their findings. That's important because excess weight has been associated with an increased breast cancer risk. The fact that women who breastfed for more than a year were less likely to be overweight than other mothers may further protect them against breast cancer.

One particularly interesting finding is that women who took drugs to dry up their milk seemed to be similarly protected from breast cancer. These drugs prevent engorgement of the breasts, which can cause inflammation of the breast tissue thought to trigger the growth of cancer cells. The researchers speculate that taking the drugs may stop that inflammation. Women who breastfeed and wean their babies gradually would also avoid this engorgement. Postpartum women who opt not to breastfeed and don't take milk-suppressing drugs usually experience painful engorgement that lasts for days before the milk dries up.

I have vivid memories of my own experiences with engorgement after I abruptly stopped nursing my 5-month-old daughter. Before I decided to call it quits, I had been pumping all her milk for several weeks because she needed to eat sitting upright owing to reflux, a gastrointestinal condition common in many babies that caused her to vomit after every meal. Now I wonder whether I caused some inflammatory changes by allowing my breasts to become engorged. I'm not too worried since I don't have a strong family history of breast cancer, but I do think that nursing moms should ask their doctors about this study finding if they're thinking of stopping cold turkey to, say, head back to work. It may be safer to taper off over several weeks--and far more comfortable, too.