Read this info from www.immihelp.com
"Results of the medical examination are valid for 1 year before you file I-485. After you file I-485, they are valid forever as long as I-485 is pending. So medical examination can be done well in advance even before I-140 is approved, without worrying about medical results being expiring(except that if I-140 processing takes much more than one year, God forbid.!)"
Hope this helps!