Zithromax refers to a group of drugs that are used against bacteria in the body. It is normally used for infections such as respiratory infections, skin infections, ear infections, and sexually transmitted diseases. It wont make you fail your drug test. So don't worry.
Zithromax is the brand name of a single macrolide antibiotic; azithromycin. (not a "group of drugs") It will not cause you to show positive on a toxicology screen. Marijuana is the likely culprit, the pregnancy is non contributory. (but please, don't smoke pot when you're pregnant).