Medical expires in 12 months and FP in 15 months but
there is a difference. As long as you have done your medical and sent your I-485 before 12 months of medical, your medical does not expire.
IN other words, medical dont expire once it is in INS hand. It only expires if you have had your medical done and you are sitting on it without filing I-485 for more than 12 months.
Offcourse, INS (BCIS) can issue RFE on medical if they are not satisfied with something and they may ask you to re-do partially or completely.