Caffeine is not supposed to be good any time in the cycle for conception. Most studies show moderation is okay. But still to be safe I have cut out coffee. Not sure if the reduced risk is cancelled out by increased crankiness...
I'll still have some caffeine in the form of chocolate and green tea. Do you drink green tea or black tea? Tea still has much less caffeine than coffee, but maybe cut back a few tea if you can?
But the book I read recently The Fertility Diet - based on a nurses study of 30,000 women showed that soda was the WORST thing you could have when TTC, worse than coffee and worse than alcohol even! It does not matter if its caffeinated or not or if its diet or not. Aspartame is terrible for TTC, and with the rest of soda I think its the acidity and the concentration of sugar.
I wouldn't worry about having a soda here and there but certainly would cut out having a daily soda if you are TTC and are not successful, sure millions of women still get pregnant while drinking soda but if it makes it take twice as long, is it worth it?