I would think one or two a day won't hurt. You can't exceed 200mg of caffeine a day while you're pregnant. One cup of Starbucks coffee sometimes exceeds that amount, but soda should be a lot less. Check the can to see how many mg's of caffeine. Good luck and take care! :-)
You shouldn't drink it all day every day or anything. But im sure a few small sips one time wont hurt. Its not crack or booze:/
I'd be more concerned about the caffeine than the sugar...