Although the safety of artificial sweeteners, like aspartame and sucralose, is a hot topic, most doctors agree that these are safe during pregnancy when used in moderation, although it's always a good idea to use sugar or other natural sweeteners whenever possible.
Generally speaking, you should aim for no more than two to four servings a day of foods and drinks that contain artificial sweeteners (like a cup of tea with Equal with breakfast and a diet soda with lunch, for example). These guidelines will also help cut down your consumption of caffeine -- an added bonus.
One exception: Moms-to-be with a condition called phenylketonuria (PKU) should not eat foods and drinks with aspartame during pregnancy. (Women with this condition can't break down aspartame properly, which can ultimately lead to birth defects.)
And all expectant moms should steer clear of saccharin (the sweetener found primarily in Sweet 'N Low) during pregnancy. Although it hasn't been linked to birth defects directly, studies have shown that it may increase an unborn baby's chances of developing bladder cancer. However, this risk is pretty slim, so if you were still sprinkling Sweet 'N Low in your coffee before you knew you were pregnant, don't worry too much -- just stop using it now.