NEW YORK (Reuters Health) - Contraception, or different reproductive patterns related to contraceptive methods, may affect the risk of some types of cancer, according to findings from the prospective Shanghai Women’s Health Study, reported in the International Journal of Cancer for May 15.
“As far as we know,” Dr. Xiao-Ou Shu and associates write, “the present study is the only one of its kind to investigate the risk of both reproductive and non-reproductive cancers in relation to common contraceptive methods based on a large, prospective, population-based cohort study.”
The study included 66,661 Chinese women, 40 to 70 years of age when recruited between 1997 and 2000. Overall, 19.4% had ever used oral contraceptives (OC), 44.9% had ever used an intrauterine device (IUD), and 12.4% had undergone tubal sterilization.
Dr. Shu, of Vanderbilt University Medical Center in Nashville, Tennessee, and colleagues report that during a median follow-up of 7.5 years, 2250 women were diagnosed with cancer. No contraceptive method was associated with overall cancer risk, the investigators report.
OC use was strongly associated with increased risk of gallbladder cancer (adjusted hazard ratio 2.38). For those who initiated OCs prior to age 29, there was a reduced risk of breast cancer (adjusted HR 0.68) compared with never users, among both pre- and post-menopausal women. Those whose OC use had stopped < 19 years before had an increased risk of colon cancer (adjusted HR 1.75).
There was a trend toward decreased incidence of thyroid cancer among IUD users, which was statistically significantly among those who were younger than the median age when an IUD was first inserted (adjusted HR 0.44).
Tubal sterilization was associated with increased risk of uterine body cancer (adjusted HR 2.50); the risk was highest among women who had the procedure at a younger age. Tubal sterilization was also associated with decreased risk of stomach cancer (adjusted HR 0.59) and increased risk of gallbladder cancer (adjusted HR 2.29).
Dr. Shu’s team notes that their findings were not affected by use of hormone replacement therapy. Also, there were too few incidents of ovarian cancer to determine if contraceptive use had any effect.
They also acknowledge that “chance findings are a likely explanation for some of the associations found in our study.”
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