CHICAGO (Reuters) - Pregnant women who drink two or more cups of coffee a day have twice the risk of having a miscarriage as those who avoid caffeine, U.S. researchers said on Monday.

They said the study provides strong evidence that high doses of caffeine during pregnancy -- 200 milligrams or more per day or the equivalent of two cups of coffee -- significantly increase the risk of miscarriage.

And they said the research may finally put to rest conflicting reports about the link between caffeine consumption and miscarriage.

"Women who are pregnant or are actively seeking to become pregnant should stop drinking coffee for three months or hopefully throughout pregnancy," said Dr. De-Kun Li of Kaiser Permanente Division of Research, whose study appears in the American Journal of Obstetrics and Gynecology.

"There has been a lot of uncertainty about this," Li said in a telephone interview. "There was no firm advice from professional societies to say what a pregnant woman should do about caffeine intake."

Li said anywhere from 15 to 18 studies have found a link between caffeine use during pregnancy and miscarriage. But that association has been clouded by the fact that many pregnant women avoid caffeine because it makes them nauseated, which could skew the results.

Li and colleagues took pains to control for that possibility. Their study involved 1,063 pregnant women who were members of the Kaiser Permanente health plan in San Francisco from October 1996 through October 1998. Women in the group never changed their caffeine consumption during pregnancy.

What they found was women who consumed the equivalent of two or more cups of regular coffee or five 12-ounce cans of caffeinated soda -- were twice as likely to miscarry as pregnant women who avoided caffeine.

This risk appeared to be related to the caffeine, rather than other chemicals in coffee, because they also saw an increased risk when the caffeine was consumed in soda, tea, and hot chocolate.

Li said many researchers think caffeine is harmful because it stresses the fetus' immature metabolism. It may also decrease blood flow in the placenta, which could harm the fetus.

"To me, the safe dose is zero," Li said. "If you really have to drink coffee, try to limit it to one cup or at the most two cups." Or better yet, switch to decaffeinated beverages, he added.

Based on the findings, Dr. Tracy Flanagan, director of women's health at Kaiser Permanente Northern California, said pregnant women should think about limiting coffee to one cup a day, and they might want to cut it out entirely.

"So many causes of miscarriage are not controllable," she said in a telephone interview. "This is an opportunity to do something active."