A new literature review published in BMJ Evidence-Based Medicine states that women should not consume any caffeine during pregnancy to avoid miscarriage and other poor pregnancy outcomes—but the meta-analysis has drawn criticism from experts in reproductive health.

Cheat sheets: Evidence-based medicine 101

The findings counter recommendations from the American College of Obstetricians and Gynecologists (ACOG) that say pregnant women can consume less than 200 milligrams of caffeine daily because it "does not appear to be a major contributing factor in miscarriage or preterm birth." Similar guidance stands in Europe and the United Kingdom, The Guardian reports.

Study details

For the review, Jack James, a psychology professor at Reykjavik University, assessed of 37 observational studies and meta-analyses published on the link between caffeine consumption and pregnancy outcomes over the past 20 years. Specifically, James examined the link between caffeine consumption and six negative pregnancy outcomes, including acute leukemia during childhood, being overweight or obese as a child, low birth weight or small size for gestational age, miscarriage, preterm birth, and stillbirth.

James found that the past studies indicate moderate to high consumption of caffeine is linked to all of those negative outcomes aside from preterm birth. "The majority of relevant peer-reviewed studies report that caffeine is associated with increased risk of negative pregnancy outcomes, including miscarriage, stillbirth, lower birth weight, small for gestational age, childhood acute leukemia, and childhood overweight and obesity," James said.

James contending that the findings indicate current clinical guidance on the topic are in need of "radical revision," and concluded, "Certainly, there is no evidence to suggest that caffeine benefits either mother or baby. Therefore, even if the evidence were merely suggestive, and in reality it is much stronger than that, the case for recommending caffeine be avoided during pregnancy is thoroughly compelling."

Critics argue study doesn't establish causality

Critics of the study argued that, because the all of the studies analyzed in James' researcher were observational, James' meta-analysis doesn't establish a causal relationship between caffeine consumption and negative pregnancy outcomes. "The harmful evidence can, in part, be accounted for by other associated factors that go with high caffeine intake, such as cigarette smoking," said Andrew Shennan, a professor of obstetrics at King's College London.

And Stacy Beck—an assistant professor in the department of obstetrics, gynecology, and reproductive sciences, maternal fetal medicine at the University of Pittsburgh—added, "Nothing in this article is really different from anything we knew. Almost all of the studies are older studies, they are retrospective studies. They looked back in time and talked to women about caffeine consumption."

Because the studies were retrospective, Beck noted, that means women included in the studies self-reported their caffeine intake after their pregnancy was over—a method that's inherently subject to human error. She pointed out that while most people know caffeine is in coffee or tea, many don't know it's in other common products, which means women likely underreported their caffeine consumption.

In addition, most of the studies in the meta-analysis also involved women who had spoken to a reproductive endocrinologist or doctor about their health before they became pregnant, which "significantly limits the type of people this data is coming from—educated women who are not low income," Beck said. "That tends to be the type of patient to talk to their doctor to review their health information. That is a huge limitation."

Emily Oster, an economics professor at Brown University, explained that, if the studies in a literature review all share similar limitations and issues—as is the case in this review—"then you're not really fixing the problem by averaging all the studies." She said, "You can say every study shows the same thing. Well, all of them have the same problem."

Adam Jacobs, associate director of biostatistics at Premier Research, also noted that James didn't disclose a conflict of interest in his study. "I note the author has published two books on the dangers of coffee, which in my opinion should have been included in declarations of interest for the journal article," Jacobs said.

However, James said he believes the research is strong enough that experts should recommend pregnant women refrain from consuming caffeine. "It is simply not plausible to suggest that current evidence implicating caffeine is so flawed as to be capable of being ignored," he said. "In fact, there is a large body of consistent evidence from well-controlled studies pointing to caffeine as a source of harm during pregnancy."

Ultimately, however, Christopher Zahn, ACOG's VP of practice activities, said there's no need for "immediate change to the current guidance" based on the review.

Oster added that, if any pregnant women are nervous after reading about the new study, "I would just go talk to your doctor … This is a place where your doctor can help you work through it. You could also read the guidelines and recognize that this is not new evidence" (Holohan, TODAY, 8/24; LaMotte, CNN, 8/24; Bakalar, New York Times, 8/26; Booth, The Guardian, 8/24).