flickr.com/photos/waytru/ 1190139568

British Medical Journal published a study of 2,635 low-risk pregnant patients, their caffeine consumption amounts, and pregnancy outcomes.  The results were pretty
interesting.   The study was performed because past studies had shown that caffeine use of more than 300 mg a day seemed to be associated with fetal growth restriction (lower weight infants) and miscarriage.  But, researchers were
unsure if past studies were done taking into account all the important
factors.

The results of the BMJ study showed that with increasing caffeine consumption, babies at birth weighed less.  For example, when compared to women who drank on average 50 mg caffeine per day those who drank 200 mg caffeine a day had babies weighing 2.5 ounces less at birth and those who drank 300 mg caffeine a
day had babies weighing 5.75 ounces less at birth.

One take-away point is that there is a small, but notable,
decrease in infant birth weight dependent on amount of maternal caffeine intake.  Researchers state that although a safe threshold cannot be determined, maternal caffeine intake of less than 100 mg per day minimizes the risk of fetal growth restriction.

http://www.bmj.com/content/337/bmj.a2332.full