NEW YORK (Reuters Health) - A longer interval between the age a woman first begins to menstruate and her age when she first gives birth is associated with an increased risk of breast cancer, the results of a study published in the American Journal of Epidemiology suggests.
Age at menstruation and first birth are "established risk factors for breast cancer," Dr. Christopher I. Li, of the Fred Hutchinson Cancer Research Center, Seattle, and colleagues write. The interval between these ages may also influence breast cancer risk because the breast becomes more susceptible to carcinogenic exposure during this period period, they note. "However, few investigators have studied this relation."
To investigate further, Li's group used data from the Women's Contraceptive and Reproductive Experiences Study (1994 to 1998), including 4,013 women with breast cancer and 4,069 women without breast cancer (the controls).
Among the white women, those who were premenopausal, had given birth and who also had an interval of at least 16 years from when they began to menstruate and their first birth had a 1.5-fold increased risk of breast cancer compared to those who had fewer than 5 years between these ages. This association was not observed among premenopausal African-American women.
The increased risks were mainly confined to women with hormone-receptor-positive tumors and tumors located in the breast lobules.
The results associating age at first full-term birth and breast cancer risk are generally consistent with previous results, but the findings regarding age at first menstruation are not, Li's team adds.
"Epidemiologic studies of both premenopausal and postmenopausal women have consistently found that breast cancer risk is reduced 5 to 20 percent for each year (menstruation) is delayed," they explain. However, in this study an older age at first (menstruation) was not associated with a reduced risk of breast cancer."
SOURCE: American Journal of Epidemiology, January 2008.
Research has shown that breast cancer risk is lower in women who are younger than 25 when they have their first child. Research has also shown that breast cancer risk is lower in women whose periods started at an older than average age.
Instead of looking at age, the study reviewed here looked at the length of time between when a woman started having periods and when her first child was born. The study showed that the more time between the two events, the higher the risk of breast cancer.
Women who had their first child 16 or more years after their first period were 1.5 times more likely to be diagnosed with breast cancer than those who had their first child less than 5 years after their first period. The researchers aren't sure why this happened, but they think that hormones might play a role.
A woman's age when she started having periods didn't affect breast cancer risk in this study.
Other research has shown that women who breast feed have a lower risk of breast cancer, no matter how old they were when they had their first child.
For both personal and practical reasons, having a first child when a woman is 25 or older is common in the United States. Concerns about the increased breast cancer risk associated with that timing may be outweighed by these personal and practical reasons. And it's also important to remember that there are many other lifestyle choices that can influence breast cancer risk and overall health. To learn more, visit the breastcancer.org Lower Your Risk section.
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