For years, obstetricians thought that even moderate caffeine consumption increased the risk of miscarriage. In fact, one study released in early 2008 found that drinking two cups of coffee a day or five cans of caffeinated soda (both of which contain about 200 milligrams of caffeine) could double a pregnant woman’s risk of miscarriage.
But a review of existing studies on the topic, released by the American College of Obstetricians and Gynecologists in August 2010, found that one cup of caffeinated coffee or one caffeinated soft drink a day won’t raise the risk of miscarriage or preterm birth.
"This is welcome news," says Gene Burkett, MD, professor of obstetrics and gynecology at University of Miami's Miller School of Medicine. "Frankly, there is no credible data that says drinking a cup of coffee a day increases the risk of miscarriage. That effect is only seen after more than two cups per day."
How big can that cup be? Think of it this way: An 8-ounce cup of brewed, drip coffee has about 137 milligrams of caffeine (instant coffee and a single shot of espresso both have about half of that). So a 12-ounce cup has 200 mg of caffeine, which should be your upper limit.