At last, a health benefit to having children late in life - it seems to reduce the risk of ovarian cancer, U.S. researchers reported on Wednesday.

They found that women who had their last children after the age of 35 had a 58 percent lower risk of ovarian cancer compared with women who had never had a child.

Women who had children earlier in life also had a lower risk, but it was less dramatic - 16 percent for women whose children were born before age 25, for example, and 45 percent for women whose children were born before age 30.

Women who had four or more children had a 64 percent lower risk than women who had never given birth, Malcolm Pike of the University of Southern California and colleagues reported in Wednesday’s issue of the journal Fertility & Sterility.

Pike’s team interviewed 477 ovarian cancer patients and 660 healthy women of similar race, ethnicity, age, and neighborhood.

The women who had babies later in life were much less likely to have had ovarian cancer, they found.

“We asked was it true for women who only had one baby, was it true for women who only had two babies,” Pike said in a telephone interview. The number of children did not matter.

“We found it was pretty consistent.”

Earlier studies have shown that having children late in life also protects against cancer of the endometrium, the lining of the uterus, said Pike.

He believes that the surge in the hormone progesterone that is seen in pregnancy may be a factor in both cancers.

“This level of progestins might very well be fatal to early disease,” Pike said.

In addition, the uterus is “cleaned out” with birth and the delivery of the placenta, perhaps taking away aging cells that are more likely to become cancerous, Pike said.

Pike believes the findings could have implications for preventing ovarian cancer, which, while rare, is deadly. “If you could work this out you could possibly do some prevention,” he said.

Dr. Robert Schenken, president-elect of the American Society for Reproductive Medicine, which publishes the journal, agrees.

“The next challenge is to map out the mechanism of the last birth’s effect on the ovaries. It would be a major advance in cancer prevention if, as the authors suggest, these findings lead to the development of a chemoprevention approach for women at high risk for ovarian cancer,” he said in a statement.

SOURCE: Fertility and Sterility, July 2004.