I did not tell my work until october and that was maybe 32 weeks
I'm not sure that it is an obligation to tell your boss unless you will have extra restrictions for your job in which case they should know. However, with a twin pregnancy anything can happen, or nothing. Sometimes it would be nice to let your boss know that there is a chance you will be put on leave early and talk to them about the process and get the paperwork sooner than later. I'm sure it's a personal choice though. Well, good luck with your twins! How far along are you and are you having fraternals or identicals?