MONDAY, Jan. 24 (HealthDay News) -- Heavy smokers of childbearing age -- especially women who have not been pregnant -- may face a modest increase in their risk of developing breast cancer, a new study suggests.

Researchers from Brigham and Women's Hospital and Harvard Medical School in Boston found that breast cancer among pre-menopausal women was associated with greater cigarette amounts over a longer time period, including taking up the habit at a younger age.

Using data collected from the Nurses' Health Study, initiated in 1976 with funding from the U.S. National Institutes of Health, the scientists examined medical records of 111,140 women over 30 years for active smoking and 36,017 women over 24 years for secondhand smoke exposure.

About 8,700 of those women went on to develop breast cancer, the most common cancer affecting women worldwide. Pre-menopausal heavy smokers had a 6 percent higher incidence of malignancy, according to senior study author Karin Michels.

However, secondhand smoke exposure in childhood or adulthood didn't appear to elevate breast cancer risk, although the authors noted that such exposure is hard to assess. Light and moderate smoking did not seem to raise breast cancer risk either, the authors said.

"I think we confirmed the fact that smoking is not an important risk for breast cancer," Michels said. "Obviously, smoking is a very important carcinogen, and most cancers are affected by smoking. Breast cancer is probably less affected."

Those most at risk of developing breast cancer began smoking before age 18, smoked 25 or more cigarettes a day or smoked more than 35 years, according to the study, reported in the Jan. 24 issue of the Archives of Internal Medicine.

Michels, a cancer epidemiologist and associate professor of obstetrics and gynecology at Brigham and Women's Hospital, said the study's large size made it easier to clarify conflicting results from previous research.

Estrogen can fuel cancer growth, and researchers believe that smoking has anti-estrogenic effect in women, lowering the amount or activity of the hormone. Consistent with that belief is the study's finding that smoking after menopause -- when hormone levels dip dramatically -- may be associated with a slightly decreased risk of breast cancer.

"Postmenopausal women in particular have an increased risk of cardiovascular disease," Michels said. "If she adds smoking on top of that, I think it's bad. This is definitely not a license to smoke."

Dr. Mary B. Daly, director of cancer prevention and control at Fox Chase Cancer Center in Philadelphia, praised the study for using a longstanding, large number of participants.

"The study is done very carefully," Daly said. "It's an interesting field because, as the authors point out, the data so far have been conflicting."

Women's breasts are more sensitive to carcinogens before experiencing a full-term pregnancy, Daly noted, making smoking potentially more dangerous for them than for pre-menopausal women who have given birth.

"The good thing about this study is that it's not going to change anything we're going to say to people about smoking," Daly said. "In terms of public health recommendations, you still want to give the message that smoking is not healthy."

More information

To learn more about breast cancer, visit Breastcancer.org.