Lividity refers to the bluish-purple discoloration of skin after death. It is a common sign associated with livor mortis, one of the four post-mortem signs of death, along with pallor mortis, algor mortis, and rigor mortis. Livor mortis is characterized by lividity and occurs within hours after death. Lividity occurs because the heart is no longer pumping blood throughout the body and, consequently, gravity pulls the blood down, resulting in the pooling of blood at the lowest point in the body.
Lividity usually begins 30 minutes to 4 hours after death and is most pronounced 12 hours after death. Blanching, or a whitish discoloration that results when pressure is applied to the skin, can occur up to 8–12 hours after death. After this time period, however, the blood becomes fixed (or non-blanchable).
Any change in the position of the body causes blood to settle down in the newer dependent areas, referred to as 'shifting of postmortem lividity'. However, this shifting may not be possible after 6–8 hours of death, due to postmortem coagulation of blood accumulated in the dependent areas of the body. This phenomenon is termed as 'fixation of postmortem staining'. Whether or not lividity is fixed is determined by the blanching test. The pressure is applied to the livid area by the thumb of the observer for about a minute, then released. If the area underneath the thumb becomes pale on the removal of pressure, the lividity is said to be not fixed, while if the region stays stained even after removal of pressure, the lividity is said to be fixed.
Lividity can also help determine the position in which an individual has died. Any part of the dead body that was in contact with a firm surface, such as the ground, during the transition from blanchable to fixed lividity, will show signs of blanching due to constant pressure pushing the blood away. For example, if an individual were to die on their back, lividity would be prominent on the back, buttocks, and backs of the legs, with blanching on the portions of the body that were in direct contact with the ground. Thus, if that body is found face down, with signs of blanching on its back, it suggests to forensic scientists that the individual had likely died on their back and been moved.
From the Affidavit of Cyril H. Wecht, M.D., J.D., in Schilling v. Baldwin, 2002 WL 33004188 (E.D.Wis. 2002):
"Livor is usually evident within ½ to 2 hours after death, and it becomes fixed by 8 to 12 hours, under normal temperatures. When a body is cooled, fixation may be delayed up to 24 to 36 hours. Prior to fixation, if the body is moved to a new position, some of this blood will redistribute to the new dependent areas. The sooner the body is moved after death, the more blood will redistribute. However, if movement is delayed until almost the time of fixation, then little will redistribute."