Generally, pregnant women can drink Diet Coke, which contains caffeine, in moderation. 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. Diet Coke contains the artificial sweetner aspartame, which 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 according to the American Pregnancy Association.