Blood clots form due to sluggish blood flow, resulting from sitting still or lying down for a long time, such as on an airplane ride or with prolonged bed rest following surgery or injury. In addition, clotting factors — substances in the blood that regulate the formation of clots — may increase after an operation or injury or during pregnancy. Other risk factors associated with the likelihood of developing DVT include obesity, a trauma (fractures, heart, hip or obstetric/ gynecologic surgery) and the use of medications such as
birth control pills and estrogen.