I drank Diet Coke and Diet Rite while I was pregnant. According to the research I did, aspertame and sucrulose (Splenda) are generally considered safe for pregnancy, while saccharine (Sweet'n'Low) is questionable.
I know that the safety of artificial sweeteners has been debated, and not everyone would agree that it's 100% safe to consume aspartame, so you have to do what you're most comfortable with. I limited my consumption to one or two drinks a week.