Most doctors recommend that during pregnancy, you should limit yourself to only one or two cups of coffee or caffeinated beverages a day. But recently, a study in mice showed that caffeine exposure during pregnancy negatively affected fetal brain development. Researchers found that mouse pups exposed to caffeine (at a dose equivalent to 3-4 cups of coffee/day for a human) showed a significant delay in migration of important neurons- similar to the effects seen in cocaine and amphetamine exposures. Long-term effects were also seen- these pups performed badly on memory tests and were more susceptible to seizures as they aged.
There are no studies to date showing evidence that caffeine exposure harms fetal growth, but this is possibly because 1) it is difficult to differentiate between the effects of caffeine versus other variables in pregnant women, and 2) women aren’t going to volunteer to consume high doses of a drug and then undergo invasive testing on their babies. Environmental factors throughout the child’s life- as well as genetic factors- also need to be considered, so it is extremely difficult to conclusively prove long-term effects in humans.
That being said, maybe expectant moms should listen to the mice and limit caffeine exposure- your kids may thank you for it later!