Women who give birth in their late 30s have a lower risk of developing ovarian cancer, research suggests.
University of Southern California scientists found women who gave birth after the age of 35 had a 58% lower risk than those who never had a child.
Women who had children earlier in life also had a lower risk - but the effect was less pronounced. Giving birth before 25 cut the risk by 16%.
The research is published in the journal Fertility and Sterility.
It also found that women who gave birth before the age of 30 had a 45% lower risk.
And women who had four or more children had a 64% lower risk than women who had never given birth.
The researchers interviewed 477 ovarian cancer patients and 660 healthy women.
Researcher Dr Malcolm Pike said previous research has also shown that having children late in life also protects against cancer of the endometrium - the lining of the uterus.
He believes that a surge in the hormone progesterone may help protect against both types of cancer.
In addition, the birthing process probably clears the uterus of aging cells that are more likely to become cancerous, he said.
Dr Pike said that if the exact mechanism could be pinned down, it could help scientists develop ways to prevent ovarian cancer.
Dr Robert Schenken, president-elect of the American Society for Reproductive Medicine, which publishes the journal, agreed.
He said: "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."
Dr Emma Knight, of Cancer Research UK, told BBC News Online previous research had also suggested that having children may protect women against ovarian cancer.
However, she said: "This information needs to be viewed in a wider context as we know that delaying the birth of a first child increases the risk of breast cancer.
"Moreover, all these risks are relatively small and women should not be overly concerned about them.
"It will be interesting to investigate the mechanisms behind these observations, with the hope that they will lead to new ways of preventing, diagnosing and treating cancer in the future."
Nearly 7,000 women in the UK are diagnosed with ovarian cancer every year.
The lack of obvious symptoms means that many cases are diagnosed at a late stage and only around a third of women survive for five years or more after diagnosis.