Antemortem means that the body was subjected to something before death. Therefore, an antemortem clot is a clot that formed in the heart or a large vessel before death, but it wasn’t discovered until the autopsy that was performed after the occurrence of death. Whereas postmortem means that the body was subjected to something after death. Therefore, a postmortem clot is a clot that formed in the heart or a large blood vessel after death, and was subsequently discovered during the autopsy. So, quite simply, the term mortem indicates that the clot was not discovered until after death, and the terms ante- and post- indicate when the clot formed; before death or after death, respectively.
Postmortem clots are dark red, smooth and shiny. These clots are moulded to the vessels /heart chambers in jelly like fashion. They are entirely unattached to the vessels walls.
Ante-mortem clots form thrombus formation. It has dull and irregular rough or somewhat stringy surface. The color of the walls is mixture of red and grey with irregular laminated layers. Thrombus will be found attached to the wall of blood vessels (heart chambers).