Researchers adjusted the results to reflect differences in weight, exercise levels and other factors, but many specialists said they suspect weight is still mostly responsible for the results.

Weight extremes — being too thin or too fat — raises the risk of any sort of infertility, said Dr. William Gibbons, who runs a fertility clinic in Baton Rouge, La., and is president of the Society for Assisted Reproductive Technology.

Other research shows that women eating lots of low-fat dairy also eat other low-fat foods and try to lose weight, said Dr. Zev Rosenwaks, director of Weill Cornell University and New York Presbyterian's fertility services.

His interpretation of the new study: "It's not that having high fat is protective. It's that being on a diet may be bad for reproduction."

The Harvard study, funded by the university and a long-running federal study, involved 18,555 women, ages 24 to 42, who became pregnant or tried to from 1991-99. Among them, 3,430 reported infertility, including 2,165 who saw a doctor for it. Of those, 438 said an ovulation problem was to blame.

The women filled out questionnaires every two years on what they ate and how often. Those who ate more high-fat dairy foods were more likely to consume alcohol and to already have had a child, and less likely to exercise than those eating low-fat dairy products. Researchers said they adjusted for these factors and still saw the link to ovulation-related infertility.

If women do eat more high-fat dairy foods to try to boost their odds of conceiving, it would be important to cut calories elsewhere to avoid gaining weight, doctors said. They also should switch back to low-fat dairy foods once the baby is born, to limit saturated fats.

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On the Net: www.oxfordjournals.org/eshre/press-release/freepdf/dem019.pdf