Giving Birth After Age 35 Cuts Cancer Risk

Women who have their last child after age 35 cut their risk of ovarian cancer in half, a new study shows.

Each additional birth before the last cuts a woman's cancer risk by about 10%. But this risk reduction is miniscule compared with that offered by having the last (or first) birth after age 35.

The findings come from a study of 475 women with ovarian cancer and 660 age- and race/ethnicity-matched women without cancer.

Compared with a woman who had no children "the overall risk of ovarian cancer in a woman with a last birth after age 35 years was reduced by 58% with a reduction of 51% due solely to the last birth," Pike and colleagues write.

Why this happens isn't at all clear. Pike and colleagues note that a woman carrying a pregnancy to term has prolonged high levels of a hormone called progesterone. They suggest that this may kill precancerous cells of the ovary or make them stop growing. If that is true, it might be possible to create a drug or hormone regimen that can prevent ovarian cancer.

That would be great, says Robert Schenken, MD, president-elect of the American Society for Reproductive Medicine.

"The next challenge is to map out the mechanism of the last birth's effect on the ovaries," Schenken says in a news release. "It would be a major advance in cancer prevention if, as [Pike and colleagues] suggest, these findings lead to the development of a chemo-prevention approach for women at high risk of ovarian cancer."

The Pike study also showed that oral contraceptive use cut ovarian cancer risk by about 6% per year of use. This effect was particularly strong for contraceptive pills containing high-dose progestins.