Women in the study who consumed more than 200 mg of caffeine a day -- the amount found in about two 8-ounce cups of regular-strength coffee -- had twice the risk of miscarriage as pregnant women who consumed no caffeine.
Researchers say the study offers some of the strongest evidence to date linking caffeine and miscarriage. But an expert who spoke to WebMD disagrees, saying the jury is still very much out on the issue.
"The evidence just isn't there to say that caffeine is a risk factor for miscarriage," says epidemiologist Lisa B. Signorello, ScD, who has studied caffeine and miscarriage. "This is a very difficult thing to investigate, and there is simply no gold-standard study that answers the question."
Caffeine and Miscarriage
The new study included 1,063 women followed from early in their pregnancies until up to 20 weeks of gestation by researchers from the Kaiser Permanente Division of Research.
The women were interviewed at study entry about their caffeine consumption and other factors known to be risk factors for miscarriage.
A total of 631 women (79%) reported reducing their caffeine consumption after becoming pregnant, while 152 (19%) said they did not change their habits.
Overall, 172 women (16%) in the study ended up having miscarriages. According to the March of Dimes, about 15% of diagnosed pregnancies end in miscarriage.
The researchers concluded that consuming more than 200 mg of caffeine a day doubled the risk of miscarriage, compared with consuming no caffeine at all.
Researchers cited their attempt to control for the confounding effect of caffeine aversion during pregnancy as a major strength of the study.
So women who are more likely to have a miscarriage might also be more likely to continue drinking coffee, and this could explain the link between caffeine and miscarriage seen in earlier studies.
But Signorello says there were not enough women in the study who did not change their caffeine consumption to conclude that caffeine was a risk factor in the miscarriage.
And the women who consumed the most caffeine were also more likely to have other risk factors for miscarriage, including being over 35 years old, having a history of miscarriage, having no morning sickness symptoms, and smoking and drinking alcohol.
Signorello is a researcher and epidemiologist with the Vanderbilt-Ingram Cancer Center and the International Epidemiology Institute in Rockville, Md.
"These women were very different from people who didn't consume caffeine, and they were different in all the ways that are associated with miscarriage," she tells WebMD.
Most Miscarriages Unavoidable
Kaiser Permanente Director of Women's Health Tracy Flanagan, MD, tells WebMD that she does discuss limiting caffeine with her pregnant patients.
She says that other than not smoking and avoiding drugs and alcohol, there are few lifestyle interventions that have been shown to affect miscarriage risk.
"The fact is, the vast majority of pregnancies that miscarry do so because of chromosomal abnormalities," she says. "From a clinician's standpoint, it is nice to be able to tell a patient that limiting caffeine just may positively impact their pregnancy."
It is a message that financial planner Tammy Plotkin-Oren took to heart after her first pregnancy ended in a miscarriage.
"I was a big coffee drinker, but I went totally cold turkey when I got pregnant again," she says.
Now the mother of three young girls, Plotkin-Oren says she has no idea if drinking coffee contributed to her miscarriage.
"For me, giving up caffeine was a no-brainer," she says. "I knew it was something I needed to stop during pregnancy. Whether or not it made a difference, I don't know."