It is said that the caffeine can make your's and the baby's heart race. When you are pregnant your blood volume increases as much as 30%, and this makes your heart have to work harder. With the already increased work load on your heart, and the rest of your system, caffeine only adds to it.
I, myself, am an avid mt. Dew drinker. When I got pregnant I limited myself to no more than 20 oz. A day. I usually have this first thing in the morning to help keep me going. I tried cutting it out ocmpletely, but I got horrible headaches.