Are diet drinks harmful during pregnancy?
I have to ask: compared to what? In normal amounts, diet drinks are not dangerous, but water and nonfat milk are probably better choices most of the time. If you are going to go for a soft drink, diet may be better, particularly if you are gaining excess weight.
You will need to read the label to make the best choices. Sucralose, the sweetener in Splenda®, is made from regular sugar. It tastes like sugar in your mouth, but the body doesn't recognize it and not much is absorbed. The FDA classifies sucralose as safe for use in pregnancy. Aspartame, the sweetener in Equal® and NutraSweet®, has also been shown to be safe in pregnancy. In the past it was believed that if the fetus had the metabolic disorder PKU, aspartame might be dangerous, but since the mom breaks the aspartame down before it gets to the baby, aspartame is considered safe to use. There isn't much information about the fetal effects of saccharine (the sweetener in Sweet'N Low®), so most experts recommend avoiding it.
As for soft drinks in general, it is best to limit or avoid caffeine during pregnancy. Juices and regular soft drinks are quite high in calories. Overall most moms are better off eating fruit than drinking juice. If you want a diet carbonated soft drink, choose one with sucralose or aspartame. Better yet, go for water or low-fat milk when you can, and save those soft drinks (diet or not) for a special treat. Everything in safest in moderation!