According to Whattoexpect.com:

Sucralose (Splenda): It's sugar, sort of. At least it starts out life that way, before being chemically processed into a form that your body won't be able to absorb, making it sweet revenge (it's calorie-free). Sucralose, which has less of that aftertaste that gives sweeteners a bad name, appears to be safe during pregnancy and has been approved by the FDA for pregnant women to consume — so sweeten your day (and your coffee, tea, yogurt, and smoothies) with it if you want. It's also stable for cooking and baking (unlike aspartame), making that sugar-free chocolate cake less pipe dream, more possibility. Look before you leap to load up on foods that are sweetened with it though; they may contain other less innocuous chemicals — or just might not be over-all nutritious choices.

Aspartame (Equal, NutraSweet): Many experts think it's harmless, others think it's an unsafe artificial sweetener, pregnant or not. But this sweetener is FDA approved for pregnant women, though they do recommend you limit your consumption of aspartame during pregnancy. A packet or two of the blue stuff now and then, a can of diet Coke every once in a while — no problem. Just avoid consuming aspartame during pregnancy in large amounts (so yes, a small piece of sugarless gum is safe), and steer clear of it altogether if PKU is on your medical chart.

Saccharin (Sweet'N Low): The FDA has deemed saccharin safe, but there have been some questionable studies (though those rat studies you may have heard of never proved much — they were never replicated in humans). But other studies have suggested that saccharin gets to your baby through the placenta, and when it gets there, it's slow to leave. For that reason, you might want to stay away from the pink packets — or pick them up only occasionally (when there's no yellow in sight).

Acesulfame-K (Sunnette): This substitute is also FDA-approved and has 200 times the sweetness of regular sugar. You'll find it in baked goods, gelatin, gum, and soft drinks, but again — moderation is key.

Sorbitol: Sorbitol is actually a nutritive sweetener, which is fine for women during pregnancy. But while it can't hurt your baby, it can have unpleasant gastro effects on you: In large doses, it can cause stomach upset and diarrhea, something no pregnant woman wants to have (diarrhea during pregnancy, besides being uncomfortable, can interfere with the absorption of vital nutrients, plus lead to dehydration). It's safe in moderate amounts but can lead to excess pregnancy weight gain if you overdo it. Sorbitol has more calories than other substitutes and less sweetness than regular sugar.

Mannitol: Like sorbitol, it's a nutritive sweetener that's safe for pregnant women, and moderate amounts are fine, but its poor absorption by your body means it can cause unfortunate goings-on in your stomach.

Xylitol: A sugar alcohol derived from plants (it’s naturally occurring in many fruits and veggies), xylitol can be found in chewing gum, toothpaste, candies, and some foods. Considered safe during pregnancy in moderate amounts (so one pack of xylitol-sweetened gum a day is fine — but you might not want to chew through five), it has 40 percent fewer calories than sugar and has been shown to prevent tooth decay.

Stevia: The latest sugar substitute to hit the market, this sweetener is derived from a South American shrub. Stevia hasn’t been approved by the FDA as a sweetener (it’s considered a dietary supplement), and no clear research proves it’s safe during pregnancy. Your best bet is to check with your practitioner before using it.

I usually drink Diet Coke because I prefer the taste, but not during pregnancy. My OB told me that he'd rather see me drinking a glass of wine than a can of Diet Coke. It's an easy enough substitution to make and since I don't like the taste of regular Coke, I just drink less pop during pregnancy by default.

I was told by my doctor to only use stevia and avoid all other artificial sweeteners.