A variety of factors can cause pitting edema in the ankles, including leg vein blockages and congestive heart failure, explains Mayo Clinic. The condition occurs when excess fluid builds up inside the tissues, which causes pit formations on the skin when pressed.
Pitting edema in the ankles can also occur during the later phases of pregnancy, explains Physicians Desk Reference. When the uterus becomes enlarged, it creates excess pressure on the large vein that transports blood from the heart throughout the body. The excess pressure increases blood pressure in the ankles and legs, which in turn leads to edema. Liver conditions, such as cirrhosis, can also cause ankle swelling and edema.