Spector and Johnson say more research needs to be done on why the risk for childhood cancer increases with advancing maternal age. Some of the possible explanations could be age-related changes in hormonal levels during pregnancy and alterations in DNA markings in eggs that can be transmitted to the offspring. "A mechanism of inherited mutation is consistent with our finding that the maternal age effect was strongest among children diagnosed with cancer at the earliest age," Spector said.

He anticipates that such research will take on greater importance as more women delay having children until older. Statistics show the percentage of babies born to women 30 years of age or older in the United States has risen from about 18 percent in 1970 to 37 percent in 2005.

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