What is post-mortem lividity?

Q: I am a 40-year-old married woman. My son passed away two months back. He had AML M2 blood cancer. When my son died, both his ears and fingernails had turned blue in colour, while other parts of the body were fine. Why did this happen? Also, please tell me what post-mortem lividity means.

A:Post-mortem lividity or livor mortis is the reddish-blue discolouration of the body that occurs in the dependent parts of the body due to gradual gravitational flow of unclotted blood. This discolouration does not occur in areas where pressure against the skin occludes capillaries as that prevents these tiny blood vessels under the skin from filling with blood, thus preventing post-mortem lividity. Finger-nails and ears are common sites as the blood vessels are superficial and the colour change is very apparent.