I think it has something to do with age (sorry) - but before I took a trip overseas in 2001 I was told that the way we were vaccinated as children was incorrect (I'm 31) and therefor I had to have a booster
BUT - I also know that my sis that is 38 comes back with no immunity EVERY time they test her - & she's been vaccinated a MILLION times for it..before & after each pg...after the last pg she said forget it!
The only reason she put up with it as many times as she did is because my mom has a first cousin that is severly mentally impaired due to the fact that her mom got German Measles while carrying her. In my family we all realize very much what a threat that is unfortunately. It was also MUCH more common then.
If you have ANY question about taking the risk - a month is only a month & your Dr is probably being a bit conservative on the 3 months. IMO - I would get a 2nd opinion about the wait before I decided whether or not to get the shot.