Researchers have confirmed earlier studies that women who use birth control pills have a higher risk of developing breast cancer. New study findings were reported in the journal Cancer Epidemiology, Biomarkers & Prevention (Vol.11: 1375-1381).

The annual rate of breast cancer is increasing, according to the American Cancer Society (ACS). It will be diagnosed in 205,000 women in the US in 2002. This number is expected to increase.

Because the growth of breast tissue depends on a woman’s hormones, researchers have focused on hormone containing drugs as one risk factor for breast cancer. Several studies have linked hormone replacement therapy to an increased rate of breast cancer in postmenopausal women.

Recent studies have found birth control pills also increase breast cancer risk, but the evidence hasn’t been as strong as that for hormone replacement therapy. Most of the reports depended on women with breast cancer remembering whether they took the pills, when they took them, and for how long.

The Scandinavian researchers set out to find the answer by doing a prospective cohort study looking at 100,000 Norwegian and Swedish women. This type of study means that a large group of women were questioned before any of them developed breast cancer and then observed. Most researchers believe this is a more reliable method.

In 1991 and 1992, the researchers sent questionnaires to 196,000 women. Over 103,000 women responded, and these formed the basis of the study. Their ages ranged from 30 to 49.

The women were followed until 1999. Because Norway and Sweden keep accurate medical records on all their citizens, the researchers could follow the health of all the women. During that time, 1,008 of them were diagnosed with invasive breast cancer.

The researcher compared the records of those women with women who didn’t develop cancer to see if birth control pills played a role. The most pronounced effect was in women who were still using or had been recently using the pill. Their breast cancer risk was increased by 60%.

Women who stopped the pill for more than a year had an increased risk of only 20%. Longer use of the pill also increased risk, but the age when women started didn’t seem to matter.

Women with inherited mutations of the breast cancer gene BRCA1 have an even higher risk of breast cancer if they take birth control pills.

A study conducted by an international group of researchers and reported in the Dec. 4 Journal of the National Cancer Institute (Vol. 94, No. 23: 1773-1779) looked at the link between birth control pills and breast cancer in women with BRCA1 and BRCA2 mutations.

Taking birth control pills did not increase the high breast cancer rate for the women with the BRCA2 mutation. But it did for women with the BRCA1 mutation. Their already high rate of breast cancer was increased by 20%.

Although the increased risk in women without a gene mutation is a concern, because these women are young their overall risk of breast cancer is still low. In this study, only one out of every 100 women developed breast cancer.

Most breast cancer occurs in older women. Over half the women who develop breast cancer are over 62 years old.

Still, experts say it is important for young women to follow the ACS guidelines for detecting breast cancer. This means a monthly self-exam for all women, a breast exam by a health professional every three years until age 39, and then yearly, and annual mammography starting at age 40.