The animal study showed one dose of caffeine found in just two cups of coffee ingested in pregnancy may be enough to affect fetal heart development and reduce heart function over the entire lifespan of the child.

The study also found that this level of exposure was associated with increased body fat among male mice.

In a press release, the organization said although the study was conducted in mice, the biological case and effect described in the report is plausible in humans.

"Our studies raise potential concerns about caffeine exposure during very early pregnancy, but further studies are necessary to evaluate caffeine's safety during pregnancy," said Scott Rivkees, Yale's Associate Chair of Pediatric Research and a senior researcher on the study.

In the study, Rivkees tested two groups of pregnant mice in a room with normal level of oxygen and another two groups in a room with only half of the normal level of oxygen for 48 hours.   In each setting, one group received caffeine ingestion and another group saline solution as a placebo.

Under both circumstances, mice given caffeine produced embryos with a thinner layer of tissue separating some of the heart's chambers than the group receiving the placebo.

The researchers found all adult males exposed to caffeine as fetuses increased body fat about 20 percent and decreased cardiac function about 35 percent compared to mice not exposed to caffeine.

"Caffeine is everywhere: in what we drink, in what we eat, in pills that we use to relieve pain, and even in candy," said Gerald Weissmann, M.D., Editor-in-Chief of The FASEB Journal. "This report shows that despite popular notions of safety, there's one place it probably shouldn't be: in the diet of an expectant mother."