Women who use birth control pills are less likely to develop ovarian cancer later in life, a new analysis of past studies suggests.
Researchers pooled data from 24 studies and found birth control pill users had a 27 percent lower risk of being diagnosed with ovarian cancer. And longer use seemed to be tied to more protection.
"It reinforces that there is a positive relationship between the use of oral contraceptives and ovarian cancer prevention in the general public," said Dr. Laura Havrilesky, who led the study at the Duke University School of Medicine in Durham, North Carolina.
"I think it adds some scientific weight to that relationship."
However, the review paper can't prove that using oral contraception lowers a woman's risk of disease - because there could have been other, unmeasured differences between women who took birth control pills and those who didn't, researchers noted.
About one in 72 women will be diagnosed with ovarian cancer during her lifetime, according to the American Cancer Society.
The disease is often caught at an advanced stage, and the majority of women who are diagnosed will die from ovarian cancer. So researchers are eager to find ways to lower a woman's chance of developing it in the first place.
Eating a healthy diet and maintaining a normal weight may be one way to do that. Some studies have suggested that using birth control pills - which contain the hormones estrogen and progestin or progestin only - may also lower a woman's risk over the long run.
To clear up that picture, Havrilesky and her colleagues combined data from 24 studies that compared thousands of women who took birth control pills for various lengths of time, at a range of ages, with those who didn't use oral contraception.
Any use of birth controls pill was linked to a lower risk of ovarian cancer, they found. Women who were on birth control pills for 10 years or longer were half as likely to develop the disease as those who didn't use them at all, the study team reported Wednesday in Obstetrics & Gynecology.
If birth control pills themselves were responsible for that reduced risk, the researchers calculated that 185 women would have to use it for five years to prevent one case of ovarian cancer.
But, because none of the studies randomly assigned women to take birth control pills or not - each woman made the decision with her own doctor - it's not clear that the contraceptives, themselves, explain the whole cancer difference.
The researchers said there hasn't been enough time to study how the specific hormone formulations in contemporary birth control pills affect ovarian cancer risk decades down the line.
Because of that and other limitations, women should use "considerable caution" when figuring the new findings into their own personal decisions about birth control, they wrote.
What's more, other research suggests women who take birth control pills are at a higher risk of developing cervical cancer, said Eduardo Franco, head of cancer epidemiology at McGill University in Montreal, Canada.
"It is the sort of thing that requires a frank conversation between a woman and healthcare provider," Franco, who wasn't involved in the new research, told Reuters Health.
He said the findings are not surprising and that many doctors are convinced birth control pills do lower ovarian cancer risk.
"I don't think there's a question of the link," Franco said. What's important, he added, is "understanding the caveats that come with the reduced risk of ovarian cancer."
"What we've got right now may be the best evidence that we ever are able to have. I don't necessarily think that it is enough to tell a physician to have their patients use oral contraceptives solely for the purpose of preventing ovarian cancer," Havrilesky told Reuters Health.
"But I think it's enough to say this is a possible advantage in women who are considering use of oral contraceptives" for birth control or other medical reasons, she said.