However, the results of this recent study suggested that such levels of caffeine from all sources, including coffee, tea, caffeinated soda and hot chocolate, pose increased dangers to pregnant women.
"The main message for pregnant women from these findings is that they probably should consider stopping caffeine consumption during pregnancy because this research provides clearer and stronger evidence that high doses of caffeine intake during pregnancy can increase the risk of miscarriage," said De- Kun Li, an researcher with the Kaiser Division of Research and lead investigator of the study.
The new study was carried out by the Kaiser Permanente Division of Research and published this month in the current online issue of American Journal of Obstetrics and Gynecology. It was supported in part by the California Public Health Foundation.
However, the US Food and Drug Administration has classified caffeine as Generally Recognized as Safe (GRAS).
Moreover, both the March of Dimes and the Organization of Teratology Information Specialists (OTIS) state on their websites that pregnant women and women trying to get pregnant can consume up to 300mg of caffeine per day.
The study looked at 1,063 pregnant members of the Kaiser Permanente health plan in San Francisco from October 1996 to October 1998 and examined the caffeine effect among women who never changed their pattern of caffeine consumption during their pregnancy.
Women who consumed 200mg or more caffeine per day, which is equal to two or more cups of regular coffee, had twice the miscarriage risk as women who consumed no caffeine, according to the researchers.
Women who consumed up to 200mg of caffeine daily had more than 40 percent increased risk of miscarriage.
According to Li, this study controlled, for the first time, pregnancy-related symptoms of nausea, vomiting and caffeine aversion that have tended to interfere with the determination of caffeine's true effect on the risk of miscarriage.
Li said: "This study strengthens the association between caffeine and miscarriage risk because it removes speculation that the association was due to reduced caffeine intake by healthy pregnant women."
The researchers said that the increased risk of miscarriage appeared to be due to the caffeine itself and not other possible chemicals in coffee because caffeine intake from non-sources such as caffeinated soda, tea and hot chocolate showed a similar increased risk of miscarriage.
Overall, 16.2 percent of women studied miscarried. While 264 women (25 percent) reported no consumption of any caffeine containing beverages during pregnancy, 635 women (60 percent) reported 0-200 mg of caffeine intake per day, and 164 women (15 percent) had 200 mg or more of daily caffeine consumption.
Information on other potential risk factors for miscarriage, including maternal age, race, education, household income, marital status, smoking, alcohol consumption, hot tub use, exposure to magnetic fields during pregnancy and symptoms related to pregnancy were collected during the in-person interview and controlled during analysis.
However, the study has come in for criticism from some quarters. Dr Alan Leviton, professor of neurology at Harvard Medical School reviewed the study and said: "Of the women participating in this study, 59 percent of the women classified as having a miscarriage had it before enrollment. Thus, this is not a prospective study as claimed by the authors."
There have been many studies looking at the effect of caffeine during pregnancy, with the concern that caffeine can cross through the placenta to the fetus. As the fetus has an under-developed metabolic system, it may find it hard to metabolize the caffeine.
Caffeine may also influence cell development and decrease placental blood flow, leading perhaps to an adverse effect on fetal development.
Because of these concerns, and the recommended maximum caffeine intake of 300mg for pregnant women, companies such as drinks giants PepsiCo and Coca-Cola last year changed their labeling to provide the exact amount of caffeine in each serving.