While there is never a good time to start smoking, new research shows that women who light up before their first full-term pregnancy may slightly increase their risk of breast cancer after menopause.

Those who start smoking after having their first child, on the other hand, appear to be no more likely to develop this cancer than those who never smoked.

“This study suggests that breast cancer prevention needs to start in adolescence, when young women are making decisions about whether to start smoking cigarettes,” said study author Dr. Janet E. Olson, of the Mayo Clinic College of Medicine in Rochester, Minnesota. She noted that current breast cancer prevention efforts mainly target adult women.

Research has consistently shown that women who delay pregnancy until a late age have an increased risk of developing breast cancer. The reason for this is thought to be related to changes that occur in a woman’s breast tissue during pregnancy.

“Breast tissue before the first pregnancy is less developed and thought to be more vulnerable to the effects of things that can cause cancer than breast tissue after a woman has had her first child,” Olson explained. “If a woman delays pregnancy, then this ‘vulnerable’ tissue is around longer, and more able to be damaged if she is exposed to carcinogens such as those that are in cigarette smoke.”

Previous studies have examined whether cigarette smoking increases a woman’s risk of postmenopausal breast cancer, but the findings are conflicting. Some researchers reported a five-fold increased risk of the condition, while others found no increased risk.

Olson and her team investigated the association by analyzing data on participants in the Iowa Women’s Health Study, which enrolled 55- to 69 year-old women in 1986 and followed them through 1999.

Overall, 37,105 women were identified as being at risk for cancer, including 7,095 who started smoking before their first pregnancy and 4,186 who started smoking after their first pregnancy. A total of 2,017 women developed breast cancer during the study period.

Mothers who reported starting smoking before their first pregnancy were 21 percent more likely to develop breast cancer than mothers who never smoked, Olson and her colleagues report in the journal, Mayo Clinic Proceedings.

These findings remained true even when the authors took into account the women’s age at first pregnancy and the number of live births, the report indicates.

Breast cancer rates among mothers who began smoking after their first pregnancy, however, were similar to that observed among the never smokers, the researchers report.

“Our research does not mean it is healthy to start smoking after a first pregnancy,” Olson stressed. “Smoking causes many health problems and should be avoided.”

SOURCE: Mayo Clinic Proceedings, November 2005.