And, it doesn't matter if the caffeine comes from coffee, tea, soda or hot chocolate. What does matter is the amount -- the study found that when women drink more than 200 milligrams of caffeine daily, the risk of miscarriage increases twofold.
"What we found was that if women have heavy caffeine intake -- greater than 200 milligrams a day -- they have double the risk of miscarriage than women that don't have any caffeine," said one of the study's authors, Dr. De-Kun Li, a reproductive and perinatal epidemiologist in the division of research at Kaiser Permanente in Oakland, Calif.
Results of the study were published online in the January issue of the American Journal of Obstetrics and Gynecology.
Caffeine, the most frequently consumed drug in the world, crosses the placental barrier and reaches the developing fetus, according to the study. While previous studies have found an association between caffeine intake and miscarriage, it hadn't been clear whether the problem was due to the caffeine or another substance in coffee, or if it had something to do with non-coffee drinkers' lifestyles -- perhaps people who didn't drink coffee ate more fruits and vegetables, for example.
For the new study, the researchers looked at 1,063 women from the Kaiser Permanente Medical Care Program in San Francisco; they were interviewed at an average of 10 weeks' gestation. During the study period, 16 percent of the women -- 172 -- had miscarriages.
The researchers found that 25 percent of the women who miscarried reported consuming no caffeine during their pregnancy. Another 60 percent said they had up to 200 milligrams of caffeine daily, and 15 percent regularly consumed more than 200 milligrams of caffeine each day.
In addition to asking about caffeine intake, the researchers also assessed the other known risk factors for miscarriage, such as smoking, a history of previous miscarriage, alcohol use and more. The researchers also compensated for nausea and vomiting during pregnancy.
"If you have a low risk of miscarriage, the effect of caffeine tends to show more," said Li.
Li said that even among women who drank less than 200 milligrams of caffeine a day, the study found a 40 percent increased risk of miscarriage, but this finding didn't reach the level of statistical significance.
"Women shouldn't drink more than two regular cups of coffee a day, and hopefully they stop drinking totally for at least the first three months. It's not a permanent stop. If they really have to drink, limit the amount to one or two cups -- a regular cup is about seven and half ounces," Li said.
But, not every doctor is convinced that there's a direct cause-and-effect relationship between caffeine and miscarriage.
"The problem with this study is that when people miscarry, a large percentage of those miscarriages are due to genetic abnormalities, and the researchers didn't say whether these were normal or abnormal fetuses," said Dr. Laura Corio, an obstetrician and gynecologist at Mount Sinai Medical Center in New York City.
"Women are always worrying and wondering, 'What did I do?' Before we say a woman drank too much caffeine and that's why she had a miscarriage, let's see if it was an abnormal or normal pregnancy, said Corio.
"I think about 60 to 80 percent of miscarriages are due to genetic abnormalities," she added.
That said, however, Corio does advise her pregnant patients to limit caffeine consumption. "Women have a responsibility to the fetus -- no cigarettes, no alcohol and just one cup of coffee a day," she said, noting that many store-bought cups of coffee contain far too much caffeine, so a woman has to be aware of how much caffeine is in her favorite coffee.
"Have less than 200 milligrams a day," no matter what the source -- coffee, tea, cola, chocolate, etcetera, Corio advised. She said caffeine has also been linked to low birth weights and smaller head circumferences.
Li also advised limiting caffeine to less than 200 milligrams a day, especially in the early months of pregnancy and in the preconception period.
To learn more about caffeine and pregnancy, visit the American Pregnancy Association.
SOURCES: De-Kun Li, M.D., Ph.D., reproductive and perinatal epidemiologist, division of research, Kaiser Permanente, Oakland, Calif.; Laura Corio, M.D., obstetrician/gynecologist, Mount Sinai Medical Center, New York City; January 2008, American Journal of Obstetrics and Gynecology, online
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