Deep venous thrombosis is the name used to describe the formation of a blood clot in the deep veins of the leg or the pelvis. The thrombosis arises either because there is a change in the way in which the blood clots, because there is inflammation in the wall of the vein or because there is a reduction in the flow of blood through the vessel. During pregnancy there is an increase in the clotting of blood, and the enlarged uterus presses on the vessels in the pelvis and so reduces the blood flow. Deep venous thrombosis is dangerous because part of the blood clot can break free and pass through the circulation to lodge in the lungs, a condition called pulmonary embolism (described in the adult blood and circulation section). Any woman who develops pain in the leg, whether or not it is associated with swelling, would be well advised to seek medical attention. Some people are more at risk of developing blood clots because they have an inherited defect in their clotting mechanisms. Any woman with a family history of deep venous thrombosis might wish to consider having their clotting mechanisms checked. Those with a history of the formation of blood clots, who have such a defect, may benefit from treatment to thin the blood during the course of a pregnancy.