SusieQQQ, Yes, absolutely correct:
"Medical Exam: As of October 1, 2013 medical examination results for immigrant visa (IV) applicants will be valid for a maximum of six months.
Visas will be valid only until the expiration date of the medical examination results (underline included). Therefore, IV applicants should not schedule a medical examination until instructed to do so by the Immigrant Visa Unit in Frankfurt, the National Visa Center (NVC) or the Kentucky Consular Center (KCC). If the visa expires before you travel, you will need to obtain (and pay for) a new visa and medical exam."
Source:
http://germany.usembassy.gov/visa/iv/