"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.
More information
To learn more about caffeine and pregnancy, visit the American Pregnancy Association.
