Women whose mothers smoked while they were pregnant have a reduced likelihood of developing breast cancer, according to a new study.

However, “This observation certainly does not suggest that smoking is beneficial,” said Dr. William C. Strohsnitter. “The very small reduction in breast cancer would be offset by the large number of other diseases caused by cigarette smoking.”

The reason for the finding may be connected to estrogen, which is linked to breast cancer risk.

“Clinical studies show that maternal cigarette smoking reduces pregnancy estrogen levels,” Strohsnitter, of Tufts-New England Medical Center, Boston, and colleagues write in the medical journal Epidemiology. “Women prenatally exposed to maternal cigarette smoke may, therefore, have a lower breast cancer risk.”

The team’s report is based on an analysis of data from the National Cooperative DES Adenosis project, a follow-up study that examined the health outcomes in women exposed in the womb to diethylstilbestrol (DES), a drug later found to have harmful effects.

The study participant’s mothers gave information about their smoking habits during pregnancy. The investigators then compared the rates of breast cancer among some 4000 women who were or were not exposed to maternal cigarette smoke before they were born.

After adjustment for other risk factors, women exposed in the womb to cigarette smoke had approximately half the breast cancer rate as those not exposed.

The association was more apparent among women whose mothers smoked no more than 15 cigarettes per day than among those whose mothers were heavier smokers. However, there were too few heavy smokers to precisely estimate a dose-response relationship.

SOURCE: Epidemiology, May 2005.