The safety of drinking diet soda while pregnant depends on the artificial sweeteners in the diet soda and whether or not it is caffeinated. According to the American Pregnancy Association, the FDA considers the following artificial sweeteners safe to ingest in moderation during pregnancy: Rebaudioside A (Stevia), Acesulfame Potassium (Sunett), Sucralose (Splenda) and Aspartame (Equal or NutraSweet). However, Aspartame (Equal or NutraSweet) should not be ingested by pregnant women with PKU (a rare metabolic disorder), rare liver disease, or by pregnant women who have high levels of phenylalanine in their blood. The American Pregnancy Association advises to avoid artificial sweeteners containing saccharin (Sweet n'Low) and cyclamate.
Pregnant women can drink soda, which contains caffeine, in moderation so long as the soda contains permissible artificial sweeteners. There have been many opposing studies regarding the safety of caffeine intake during pregnancy. According to the American Pregnancy Association, caffeine is permitted during pregnancy but intake should be limited to under 200mg per day. However, some sources suggest to avoid all caffeinated products during the first trimester of pregnancy.