It doesn't seem likely that you are 3 weeks pregnant. Pregnancy is counted from the first day of your last menstruation, not from the day of conception. According to your last menstruation on the 22nd February, it seems that you are 5 weeks and 1 day pregnant at the moment you are asking this question (the 29th of March).


Xanax (alprazolam) is a benzodiazepine used for short term treatment of anxiety. Pregnant women should avoid Xanax because it can cause muscle weakness and respiratory problems in the newborn. Xanax (alprazolam) belongs to "Pregnancy Group D", which means it can cause congenital anomalies if used in the first trimester of pregnancy. I don’t believe that one pill of Xanax can damage your baby but you must tell your gynecologist about this. There are no specific tests that can tell us if your fetus was damaged, or not, at this stage of pregnancy. You can only perform the scheduled ultrasound scans. Your gynecologist may ask for more information, if necessary.