The study is a strong indicator of the continued need for smoking prevention messages to all, but especially ones tailored to this group of young women.
"Considering the young average age at smoking initiation, this study provides further justification for smoking prevention efforts aimed at young women," says Janet Olson, Ph.D., a Mayo Clinic Rochester researcher and the lead author of the Mayo Clinic Proceedings study.
The risk estimates for women who smoked before their first pregnancy were 20 percent higher than those of nonsmokers. These levels of risk elevation are consistent with the risk levels reported from other epidemiological studies of cigarette smoking and breast cancer risk, says Dr. Olson. Women who started smoking after their first pregnancy had rates of breast cancer similar to those women who never smoked.
The researchers did not find evidence that duration of smoking or number of cigarettes smoked per day affected risk of breast cancer among the smokers.
Scientists have known for many years that women who are young (under age 20) when they have their first baby are less likely to get breast cancer than women who are older (over the age of 35) at the birth of their first child. It is not known exactly why this is true, but it is thought to be linked to the changes that take place in women's breasts during pregnancy.
Breast cancer usually starts from cells that are most like the cells found in a woman before she has her first child. When she has her first baby, a woman's breasts change so she will be able to breastfeed her child. One theory is that the breast cells of women before they have their first baby are more likely to be damaged by things that can cause cancer, like the chemicals in cigarette smoke. If a woman has her first baby sooner rather than later, then when she is exposed to things that cause of cancer, she has fewer of the breast cells that are most likely to be easily damaged by that exposure.
The Mayo Clinic Proceedings study used data from the Iowa Women's Health Study (IWHS). The IWHS included 41,836 women from Iowa, ages 55 to 69, who responded to a questionnaire mailed in 1986. The study was designed to identify risk factors for cancer and other chronic diseases in postmenopausal women. Over 200 publications from this cohort of women have been published to date.
The research team also included: Celine Vachon, Ph.D.; Robert Vierkant; Paul Limburg, M.D.; James Cerhan, M.D., Ph.D.; all of Mayo Clinic; Carol Sweeney, Ph.D., of the Department of Family Preventive Medicine, Health Research Center, Salt Lake City, Utah; and Thomas Sellers, Ph.D., of H. Lee Moffitt Cancer Center and Research Institute, Tampa, Fla.
A peer-review journal, Mayo Clinic Proceedings publishes original articles and reviews dealing with clinical and laboratory medicine, clinical research, basic science research and clinical epidemiology. Mayo Clinic Proceedings is published monthly by Mayo Foundation for Medical Education and Research as part of its commitment to the medical education of physicians. The journal has been published for more than 75 years and has a circulation of 130,000 nationally and internationally. Copies of the articles are available online at www.mayoclinicproceedings.com.