Birth control pills have long been known to reduce the risk of ovarian cancer, but a new study suggests any type of contraceptive – even a vasectomy may also be protective.
After comparing women with ovarian cancer to those without, researchers found that women who used any type of contraception –birth control pills, tubal ligation (getting your "tubes tied"), intrauterine devices (IUDs), barrier method (such as diaphragms) or male vasectomy, had between a 40% and 65% lower risk of ever developing ovarian cancer.
It's not a surprise to see an association between birth control pills and ovarian cancer, and even to see one for tubal ligation, since other studies have shown the same thing, study author Dr Roberta Ness of the University of Texas School of Public Health said.
But to see a similar trend for women who relied on IUDs, the barrier method, or a partner's vasectomy to avoid pregnancy is somewhat surprising, Ness acknowledged.
"No matter what method of contraception you use, it looks like that would be protective."
More than 21,000 US women will develop ovarian cancer in 2010, and another 14,000 will die from it. Approximately 1.4% of women will develop ovarian cancer in their lifetimes.
Impact on future risks
Given that some forms of contraception are associated with a lower risk of ovarian cancer, Ness and her team set out to see whether all types had any impact on the future risk of disease.
They interviewed 869 women who had developed ovarian cancer, and 1779 others who had not developed the disease, about their history of contraception. Women were considered to use no artificial contraception if they relied on natural family planning (avoiding sex during ovulation) or withdrawal.
As expected, women who had gotten their tubes tied or taken birth control pills in the past, either for contraception or other reasons, were less likely to have developed ovarian cancer, they reported in the journal Annals of Epidemiology.
But strangely, so were women who used an IUD. Specifically, women who developed ovarian cancer were less likely to have ever used an IUD –among those with cancer, only 12% had ever used an IUD, versus 17% of women who did not develop the cancer.
Similarly, 14% of those with ovarian cancer had a partner who had undergone a vasectomy, but so did 17% of the women who never developed ovarian cancer.
After using statistical tools to remove the influence of factors that could affect the relationship between contraception and cancer, Ness and her team found that relying on a vasectomy or IUD lowered women's risks of ovarian cancer by 50% and 60%, respectively, relative to women with no history of artificial contraception.
It's possible that sperm itself may increase inflammation when inside a woman, and this inflammation could, over time, increase her risk of ovarian cancer, Ness suggested. Any technique that reduces a woman's exposure to sperm; vasectomy, tubal ligation, or IUDs, would therefore potentially protect women from that inflammation, she said.
Birth control pills also affect inflammation, which may explain their benefits, too, Ness added. Oral contraceptives reduce the number of times a woman ovulates, and each ovulation is associated with an increase in inflammation.
Women who have given birth to children also have a lower risk of ovarian cancer, however, and when the authors factored in the influence of pregnancies, they saw a much weaker relationship between vasectomy, IUDs and the barrier method.
It's not surprising that the relationship got weaker, Ness said, since dividing women by both pregnancies and contraceptive history made each category very small. "I don't actually think that negates the finding."
But Shelley Tworoger at Harvard University, who reviewed the findings for Reuters Health, disagreed. "I don't really believe vasectomy is associated with lower ovarian cancer risk, because it happens in the man." Alternatively, there may be some important differences in the lifestyles of couples who choose vasectomy that in turn lower women's cancer risk, she suggested.
Since women were asked to recall their history of contraceptives, it's always possible they could mis-remember something, but that's unlikely, Tworoger added. "With something like contraception, we know people's memories are actually pretty accurate."
What's always consistent across research, Ness added, is the relationship between ovarian cancer and birth control pills, suggesting that a woman deciding on contraception should consider the pill's impact on her future risk of this "highly fatal" form of cancer. "I think this ought to be at least part of the conversation when women are making that contraception choice."
(Reuters Health, Alison McCook, December 2010)