Factor V Leiden

Definition - What does Factor V Leiden mean?

Factor V Leiden is a mutation in a factor that assists with blood clotting. Factor V is a protein which makes the clotting protein thrombin. Once thrombin is created, it causes dissolved proteins in the blood to congeal into solid masses, forming a blood clot.

In people with Factor V Leiden, thrombin is more likely to be created and activated, even if there has not been an injury that requires a blood clot to stop bleeding. This means that people with Factor V Leiden are at higher risk for blood clots that form in the veins, which can be dangerous.

Women with Factor V Leiden are also at 2-3 times the risk of miscarriage in the second and third trimesters, due to the potential for blood clots to block blood supply to the baby. These women are also be at increased risk for blood clots that are dangerous to their own health during pregnancy.

Most people with Factor V Leiden do not know that they have it until they are tested for the condition. Women with a personal or family history of blood clots may be treated with anti-coagulants during pregnancy to protect against the formation of more clots.

FertilitySmarts explains Factor V Leiden

Blood clotting is mediated by a complex biochemical cascade, that is normally activated only when an injury has occurred and a clot may be needed to prevent bleeding. However, in Factor V Leiden, this cascade may sometimes be started spontaneously for no reason.

When a mutant version of the Factor V protein activates thrombin spontaneously, thrombin changes proteins and platelets in the blood, causing them to form a solid clot inside an artery or vein. This clot can block blood vessels, and can be life-threatening if it moves to block vessels leading to the lungs or brain.

Such blood clots can also block vessels which supply blood to a developing fetus, leading to miscarriage. Pregnant women are at increased risk of blood clots because the hormones estrogen and progesterone, which are elevated in pregnancy, also encourage blood clot formation.

Most people do not know they have the Factor V Leiden mutation until the occurrence of a blood clot or recurrent miscarriages causes a doctor to suspect that they may have a clotting disorder.

Doctors may do a blood test to look for signs of Factor V Leiden if a patient has had:

  • One or more dangerous blood clots
  • A series of miscarriages
  • A known family history of dangerous blood clots

Women who have Factor V Leiden and other clotting disorders are typically treated with anticoagulants, such as heparin, during pregnancy. This drastically reduces the chance of blood clot formation, and often leads to a healthy pregnancy and delivery.

Doctors may recommend treatment with heparin or another anticoagulant for pregnant women who:

  • Are known to have Factor V Leiden
  • Have had one or more blood clots in the past
  • Have a family history of blood clot formation


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