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Nov. 24, 1999 (New York) -- Pregnant women who can't seem to give up their morning cup of coffee or tea may be interested in the results of a new study showing that moderate caffeine consumption does not increase the risk of miscarriage. Although some studies have indicated that caffeine could increase the risk of miscarriage, this study, appearing in the Nov. 25 issue of The New England Journal of Medicine, found that rates were only affected in women who consumed caffeine in amounts equivalent to more than five cups of coffee per day.
The study consisted of blood samples obtained from pregnant women enrolled in the Collaborative Perinatal Project, which was conducted from 1959 to 1966. Serum paraxanthine, the primary by-product of caffeine, was measured in almost 600 women who had miscarriages at less than 140 days into their pregnancy. They compared them to nearly 2,600 women who gave birth to live infants at 28 weeks' gestation or later and had blood drawn on the same day of gestation as the women who miscarried. They found the mean serum paraxanthine concentration was higher in the women who had miscarriages than in the other women.
"The reason we decided to use this particular database was because per-capita coffee consumption in the U.S. peaked around 1962, so it was convenient that we were able to access a database of pregnant women who were pregnant predominantly in the early 1960s," the study's lead author, Mark A. Klebanoff, MD, MPH, tells WebMD. "More recent studies have actually had a hard time recruiting large numbers of women who consumed large amounts of caffeine while pregnant." Klebanoff is a medical officer at the National Institute of Child Health and Human Development, in Bethesda, Md.
Women with paraxanthine levels higher than 1,845 nanograms per milliliter of blood had the greatest increased risk of miscarriage. Those with concentrations above 1,845 nanograms per milliliter had nearly double the risk.
Klebanoff says a conservative estimate would be that more than five cups of coffee per day would be needed to achieve a blood level of 1,845 nanograms per milliliter or higher. A typical cup of brewed coffee has about 100 milligrams of caffeine and a cup of tea has about 40 milligrams.
Caffeine is a stimulant that crosses the placenta, meaning it easily passes from mother to child. Based largely on data from animal studies showing harmful effects of caffeine on fetuses, the FDA has advised since 1981 that pregnant women "avoid caffeine-containing foods and drugs, if possible, or consume them only sparingly."
But as Brenda Eskinazi, PhD, points out in an editorial accompanying Klebanoff's study, most pregnant women do not heed this advice. "In part because of our own dependence on our morning cup of coffee, and because of our inability to find strong associations with effects on health in humans, we have accepted that more than 75% of pregnant women consume caffeinated beverages," writes Eskinazi, of the University of California School of Public Health, Berkeley. She says health care providers should continue to counsel women who are pregnant or breast-feeding to limit their caffeine intake. Eskinazi also points out that in addition to the risk of miscarriage, caffeine has been linked in animal and human studies to changes in fetal heart rate and breathing patterns, decreased brain weight and alterations in brain development, learning and memory.
"We only looked at risk of spontaneous abortions in this study and we didn't see any increased risk until women were consuming very large amounts of caffeine," Klebanoff says. "So it seems to me that the best advice is that moderation in all things is always a good idea, but there are probably more important things for pregnant women to worry about than whether they are having a cup of coffee or two a day or a cup of tea or two a day."
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