"Since when are you (so)...?" tends to be pretty rude - it can mean you don't really believe them. "Since when are you pregnant?" probably means that you saw or talked to them recently, and they didn't tell you (or it wasn't obvious), and that you're angry at them for finding out later. "Since when are you so considerate?" means that you are shocked that this person thinks they are considerate, because you have never thought of them that way. It might be that you see them being considerate to another person in front of you, and you're upset that they don't treat you the same way.

How long have you been pregnant?/How far along are you in your pregnancy? When did you become so considerate?