While I can't say much good about consuming artificial sugar substitutes (or artificial sweeteners) while you're pregnant, I can't say much bad about them either. While the evidence doesn't show that they're harmful to developing fetuses, I would simply make the point that virtually everything can pass through the placenta, so why would you want to muck up your baby's blood with a bunch of chemicals?
I know you want to watch calories and avoid sugar, and that's good. But now can also be a time to be creative in the kitchen and find ways of avoiding both sugar and artificial sweeteners. For example, in many cases, fruit juice can be a nice substitute that helps curb your craving for sweets without pumping you full of chemicals or filling you with fat. Another good option: Agave nectar, which isn't fermented; otherwise, it's called tequila. Unfermented, it has a cup of sweetness for a half teaspoon of calories.
Find out more about this book:YOU: Having a Baby: The Owner's Manual to a Happy and Healthy Pregnancy