Soda vs. diet soda

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Currently ttc but have a friend who is about 18 weeks pregnant. Her doctor recommended diet vs regular soda and no sweet tea. Is this something your doctors have recommended? Is it sugar, caffeine, etc. that would make her doctor say this? I know everything in moderation is somewhat ok just curious the reasoning behind this. I've always been under the assumption that diet sodas are worse because they still have high levels of artificial sugars and some still contain caffeine. I don't drink coffee but do drink a regular soda once every day or every other day, everything else is water only...