The medical report is only valid for 6 months. Both the visa and medical must be valid when you enter the US. So, if your medical is 2 months old when you have your interview, this will usually be reflected in the visa expiry date being 2 months less than the possible 6 month expiry. Depending in your intended timing for entry to the US, this may create a problem if you were needing 5-6 months to get ready to move. Most of us tried to get the medical as close a possible to the interview date, but early enough to ensure it was available for the interview