Giving Birth After Age 35 Cuts Cancer Risk

July 16, 2004 -- 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. Malcolm C. Pike, PhD, and colleagues at the University of Southern California, Los Angeles, report the findings in the July issue of Fertility and Sterility.

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.

SOURCES: Pike, M.C. Fertility and Sterility, July 2004; vol 82: pp 186-195. News release, American Society for Reproductive Medicine.