there was a study recently in to the affects of caffiene in first tri. i think, though im not certain and dont have the time to look up the study atm, it said there was a slight increase in risk of miscarriage when the mother was drinking large amounts of caffiene.
it was larger amounts so i wouldn't worry if its the odd glass of coke, maybe try the caffiene free stuff.
i loved coke when pregnant but it had to the the sugary caffiene loaded stuff and mt craving didnt start until 2nd tri, watch out later on mind cos caffiene used to keep Tally up kicking al night