I need help!

So my brother has a girlfriend that he has been on and off with for over 4 years now. They seem really serious now and today she told me that she went to the doctor and told him that her and her boyfriend have been ttc for two years now (they haven't, my brother doesn't want kids right now because he has no job) the doctor gave her preseed which I guess will help her get pregnant. She told me not to tell my brother she wants to surprise him when she gets pregnant. I know I should tell him but I don't know how to because all I can think of saying is " hey your gf is psycho and is trying to get pregnant knowing you don't want a child right now" but that seems mean.

Any ideas on how to tell him??