Cup of Contradictions

In the age of Starbucks venti lattes, it's tough for expectant moms to go cold turkey on caffeine. But they may want to cut back because of the possibility that the popular stimulant increases the risk of miscarriage.

A study published Monday in the American Journal of Obstetrics and Gynecology (AJOG) reports that women who reported consuming more than 200 milligrams of caffeine a day during the first three months of pregnancy doubled their risk of miscarriage (from the 12 percent average among participants who used no caffeine to 25 percent). During a two-year period, 1,063 pregnant San Francisco-area women participated in the study.

Traditionally, groups like the American College of Obstetricians and Gynecologists and the March of Dimes have told women that drinking one or two cups of coffee a day is OK for most pregnant women. But these new findings suggest otherwise. Indeed, expectant mothers might be surprised to find out how little 200 mg of caffeine is by today's mega-cup standards. For example, the smallest size of Starbucks' regular coffee, the "tall" or 12-ounce cup of brewed joe, has 260 mg of caffeine. An eight-ounce cup of regular drip coffee contains 137 mg, an eight-ounce cup of brewed tea contains 48 mg and a 12-ounce soda contains about 37 mg.

Exactly how caffeine might be affecting the developing fetus is unclear. Doctors know that that the stimulant can easily cross the placental barrier to the fetus, but whether there's a negative impact, is unclear. "That's true of most things that cause miscarriage," says Dr. Mark Nichols, an OB/GYN at Oregon Health and Sciences University.

It could be because caffeine constricts the blood vessels and restricts blood flow to the placenta, says De-Kun Li, co-author of the new study and a researcher in the division of research at Kaiser Permanente in northern California. Or it could be that caffeine interferes with developing cells' normal function. "The fetus is very sensitive to any environmental insult," says Li. Even with the ambiguity, the findings are enough to convince Li. "If it was my wife, I would say stop drinking for [the first] three months [of pregnancy]," he says.

Still, research on the link between caffeine consumption and miscarriage remains controversial. Also this month, in the journal Epidemiology, researchers reported that consuming less than about two cups of coffee a day was unrelated to miscarriage. "I am not persuaded [caffeine] poses a health hazard. We really don't know the truth," says epidemiologist David Savitz, professor of community and preventive medicine at Mount Sinai School of Medicine in New York and co-author of the Epidemiology study.