Agree with above, but as mentioned don't forget about nitrofurantoin. Hardly ever mentioned, but it concentrates in urine and has activity pretty much only there and nowhere else, and does a great job against most bladder infections. Major toxicity is to the lung, so the patient has to be aware of early symptomatology. Gonorrhea/syphilis/chlamydia gets the usual. Amoxicillin for chlamydia in pregnancy instead of azithromycin.
Syphilis really needs penicillin, and if necessary you can do a one-time desensitization process in the case of allergy, even in pregnancy. It's not something to be taken lightly though, and shouldn't be stopped once you start.