Conditions and factors that may cause blood clots include smoking, pregnancy, obesity and surgery, according to Mayo Clinic. Certain medications, such as some breast cancer drugs, oral contraceptives and hormone therapy medications, may also cause blood clots. Individuals who suffer from heart arrhythmia or who have a family history of blood clots are more likely to get blood clots.
Excessive blood clotting may occur when the body's normal blood clotting process is wrongly triggered or altered, explains the National Heart, Blood, and Lung Institute. Certain diseases cause excessive clotting or prevent the proper dissolution of blood clots. Autoimmune disorders such as antiphospholipid antibody syndrome cause the body to make antibodies that fight against phospholipids in living cells. This triggers the formation of blood clots in veins.
Bone marrow disorders may cause the body to make an excessive amount of blood cells, and this may cause blood clots to form, according to the National Heart, Blood, and Lung Institute. Atherosclerosis causes the buildup of plaque in blood vessels. After a while, the plaque raptures and platelets form blood clots at the site of the damage. Vasculitis damages blood vessels and triggers the formation of blood clots at the site of the damage. Obesity increases the risk of atherosclerosis, which may cause blood clots.
Heart failure slows down the flow of blood, and this may cause the formation of blood clots, reports the National Heart, Blood, and Lung Institute. Arrhythmia and atrial fibrillation cause blood to pool in the heart's upper chambers, resulting in the formation of clots. Smoking damages blood vessels and causes platelets to stick together. Women have more clotting factors in their blood while pregnant, and this increases their risk of blood clots.