Confusing messages are nothing new when it comes to what you should notdo during pregnancy, so here's another one: A growing body of evidence suggests that high caffeine consumption - on the order of more than two or three cups of coffee a day - increases the odds of miscarriage, while low to moderate caffeine consumption doesn't pose a significant risk.
The only trouble is, it is still not clear what "moderate" consumption means, said Dr. Mark Klebanoff, a pediatrician and epidemiologist with the National Institute of Child Health and Human Development.
Two studies published this month, which use similar approaches but yield different results, underscore that point.
A study in the journal Epidemiology found no association between low to moderate caffeine consumption and miscarriage risk. The study included 2,407 women who were interviewed about their caffeine consumption during the first 20 weeks of pregnancy.
On average, study participants consumed 350 milligrams per day of caffeine prior to and early in pregnancy, and 200 milligrams per day closer to the 20-week mark. At those end levels, equivalent to about two eight-ounce cups of coffee or six cans of cola per day, the researchers found no evidence of an elevated miscarriage risk.
Just as important as what the study concludes, though, is what the study doesn't conclude, said lead author David Savitz, an epidemiologist at the Mount Sinai School of Medicine in New York.
"We weren't addressing women who drink eight cups a day; we were addressing maybe two cups a day, so there's a narrow conclusion you can draw," he said. "It adds to the evidence suggesting that those levels of caffeine are not a concern, but doesn't address higher levels of coffee consumption."
At the other end of the spectrum, a study published today in the American Journal of Obstetrics and Gynecology suggests that higher doses of caffeine do pose an increased miscarriage risk.
The study of 1,063 women found double the risk of miscarriage when women consume more than 200 milligrams per day of caffeine. In the study, conducted by the nonprofit Kaiser Permanente Division of Research, in Oakland, Calif., 12.5 percent of women reporting no caffeine consumption during the first 20 weeks of pregnancy miscarried, versus 25.5 percent of women consuming more than 200 milligrams of caffeine per day.
That a high level of caffeine is associated with miscarriage is consistent with earlier studies, said Dr. Laura Riley, director of labor and delivery at Massachusetts General Hospital.
The paper also addresses a major flaw found in earlier studies, said Dr. Jorge Chavarro, research fellow at the Harvard School of Public Health.
Women with nausea in pregnancy have a lower risk of miscarriage than women without nausea, Chavarro said. And, nauseated women usually develop a distaste for coffee, which introduces a complication in studies of caffeine and miscarriage. Previous studies failed to account for this difference, potentially overcounting women who were likely to miscarry, regardless of whether they consumed caffeine.
"If you only compare the caffeine intake of women who did and didn't miscarry, to some extent you're comparing women who had and did not have nausea," he said.
But Klebanoff points out that the level at which this paper identified miscarriage risk - more than 200 milligrams per day - overlaps with the "moderate" level deemed not risky in the Savitz paper. While he can't explain the discrepancy, he points out that a challenge in these studies is that people's bodies process caffeine differently, so a "moderate" amount for one individual might be "high" for another.
Both of the new studies have their drawbacks, Riley said. She points out that it is hard to standardize the amount of caffeine in coffee from different retailers, and that people don't always accurately report their own behavior in surveys.
Still, she says, the evidence is compelling enough to suggest that pregnant women curb their caffeine intake. But that doesn't mean they need to go cold turkey.
"Would I cut it out completely? Probably not; the literature doesn't support the idea that a little bit of caffeine is harmful," Riley said.
