Thrombophilia

Is Testing Right for Me? I Have My Test Results

Thrombophilia

Women and Thrombophilia

All women have several possible extra risk factors for abnormal clotting. These include birth control pills, hormone replacement therapy, and pregnancy.

Women with an inherited thrombophilia may have a much higher risk of clotting when combined with one of these risk factors. Thrombophilia may also increase the risk of pregnancy complications and loss.

Birth Control Pills
Hormone Replacement Therapy
Pregnancy

Birth Control Pills

All women who use birth control pills ("the pill") have an increased risk for potentially dangerous blood clots, compared with women who do not take the pill.

If you have an inherited blood clotting disorder, like factor V Leiden or a prothrombin mutation, you have an even higher risk for abnormal clotting when you take birth control pills.

  • Any woman taking birth control pills has a 2 to 4 times higher chance of an abnormal clot.
  • A factor V Leiden mutation increases the risk an extra 16 times. Together, this means a women with a factor V Leiden mutation who also takes birth control pills has up to a 35 times higher chance for clotting.
  • Similarly, a woman who takes birth control pills and has a prothrombin mutation has an average 16 times higher risk.

Hormone Replacement Therapy

Hormone replacement therapy (HRT) is usually given to relieve the symptoms of menopause. Like oral contraceptives, all women who use HRT have an increased risk of developing a blood clot in the veins. An inherited thrombophilia seems to increase this risk much more than HRT alone.

  • Any woman on hormone replacement therapy has a 2 times higher chance of an abnormal clot.
  • Women with one factor V Leiden mutation who take HRT have a 13 to 15 times higher risk of clotting than women with neither risk factor.
  • The risk from HRT with one prothrombin mutation isn't well known. One study found an 8 times higher risk, while others have found no increased risk.

Pregnancy

Venous Thrombosis

Pregnancy increases the chance for abnormal clotting in all women. During pregnancy, thrombophilia further increases the risk for clotting and certain other complications.

  • Every pregnant woman has about a 4 times higher chance of an abnormal clot compared to women who aren't pregnant.
  • A pregnant woman with one factor V Leiden mutation has an 8 times higher chance of abnormal clotting than a pregnant woman without the mutation. Two factor V Leiden mutations increase this risk 20 to 40 times.
  • Similarly, a pregnant woman with one prothrombin mutation has about a 7 times higher chance of abnormal clotting than a pregnant woman without the mutation. The risk of two mutations isn't known, but is believed to be much higher than this.

Other Pregnancy Complications

Although studies haven't all agreed, many studies have found that thrombophilia increases the chance of certain pregnancy complications a small amount. These complications may include:

  • Preeclampsia — extremely elevated blood pressure and protein in the urine.
  • Placental abruption — when the placenta tears away from the side of the uteus, often causing heavy bleeding and lack of blood supply to the pregnancy.
  • Intrauterine growth restriction (IUGR) — when a baby is considerably smaller than expected, which may affect the health of the baby.
  • Recurrent pregnancy loss — usually defined as 3 or more unexplained pregnancy losses.
  • Late pregnancy loss — a stillbirth usually in the third trimester.

Although not proven, some researchers suspect that thrombophilia may increase the chance of blood clots in the placenta (afterbirth). Since the placenta is where oxygen and nutrients are exchanged between the mother and developing baby, clots may block the flow of these substances and cause complications.

Factor V Leiden or Prothrombin G20210A Mutations and Pregnancy Complications

Outcome One Factor V Leiden Mutation One Prothrombin G20210A Mutation
Recurrent first trimester pregnancy loss 2 times higher risk 2 to 3 times higher risk
Third trimester pregnancy loss 3 times higher risk Up to 9 times higher risk
Placental abruption Up to 5 times higher risk (but studies disagree) Up to 8 times higher risk (but studies disagree)
Preeclampsia 1.5 to 3 times higher risk (but studies disagree) 2 to 3 times higher risk (but studies disagree)
Intrauterine growth retardation Up to 3 to 5 times higher risk (but studies disagree) Up to 3 times higher risk (but studies disagree)

Next: Resources

Why are you considering genetic testing? Check only one at a time.

I have a history of one or more blood clots that could be related to an inherited thrombophilia. (See Who Should Consider Testing if you aren't sure.)

I have a family member with a factor V Leiden and/or prothrombin gene mutation.

I have a family history of abnormal blood clots that could be related to an inherited thrombophilia (usually clots before age 50 or abnormal clotting in more than one generation).

I have had serious pregnancy complications, including miscarriage, stillbirth, placental abruption, severe preeclampsia or fetal growth problems

None of these.

Testing may help explain your clot, but may not change how your doctor treats you.

Factor V Leiden (FVL) and prothrombin G20210A (PT) mutations are more common in people who have had a vein clot. In people who have had their first blood clot, 1 in 5 (20%) have an FVL mutation and 1 in 16 (6%) have a PT mutation.

You have a higher chance of having a factor V Leiden or prothrombin gene mutation if you have had any of the following:

  • A vein clot at a young age, usually before age 50
  • History of more than one vein clot
  • A clot caused by pregnancy, birth control pills, or hormone replacement therapy
  • A clot with no apparent cause (called "unprovoked" or "idiopathic")
  • A clot in an unusual place like veins in the liver, kidneys, gut, and brain

Once a person has a clot, the main concern is preventing another. Most clots are treated with blood thinners and other drugs for some period of time to prevent more clotting — no matter what the reason for the clot is.

Most people with only one mutation have a higher risk of complications from long-term blood thinners than the risk of clotting from the mutation. A factor V Leiden mutation increases your chance for another clot only a little (less than twice the average risk). The prothrombin mutation doesn't seem to increase the risk at all for most people. As a result, experts don’t recommend treating someone with one mutation for longer than usual.

People who have had a clot may be treated differently if they have other higher risk situations in the future, like surgery or pregnancy. Women are also generally advised to avoid certain oral contraceptives, hormone replacement therapy, and other drugs that have estrogen if they have other good options. But again, the treatment recommendations are based on having had a clot already — not having a mutation.

Rarely, a person will have two FVL and/or PT mutations. The risk for clotting is much higher with two mutations. However, this result is so uncommon that most experts don’t think it is worthwhile to test people with a clot to look for two mutations.

Even if the test results don’t change the drugs a person takes to prevent clotting, some people find it useful to know for other reasons. For example, there are other ways to help prevent blood clots that don’t involve medications. Some people may be more conscious of preventing a clot or recognizing it early when they know they have a mutation.

Testing may alert you to a small increased risk for clotting, but may not change how your doctor treats you.

People with a close relative (parent, child, or sibling) who had a blood clot also have a higher risk for clotting — about 2 to 3 times higher than average. A family history of clots at a young age or more than one relative with a clot could mean that a factor V Leiden (FVL) or prothrombin (PT) mutation runs in your family. There are inherited causes of clotting, but they are less common. You should talk to your doctor or a genetic counselor about your family history to understand your risks and whether any testing may be useful.

You have a higher chance of having a factor V Leiden (FVL) or prothrombin (PT) gene mutation if one of your blood relatives has a mutation. Your actual risk depends on how closely you are related to this family member. If you aren’t sure already, try to find out the name of the mutation that runs in your family. If you decide to have testing, this information will help your doctor be sure you get the right test.

Factor V Leiden and prothrombin mutations increase the chance of clotting. The average person has about a 1 in 1000 chance for an abnormal clot each year. People with a FVL mutation have a 3 to 8 times higher chance that average. People with a PT mutation have a 2 to 4 times higher chance. This still means that the chance for a clot is relatively low — no more than a 1 in 125 to 1 in 500 chance each year.

People may also have two FVL and/or PT mutations. Two mutations raises the risk for clotting much more, but is rare.

People who have had a clot, or who have a very high risk, may be treated with drugs that thin the blood, called anticoagulants. However, these drugs have risks. Most people with only one mutation have a higher risk of complications from long-term blood thinners than the risk of clotting from the mutation. Most experts feel that finding a FVL or PT mutation should not change the treatment of someone who has never had a clot.

When people know they have a higher risk for clotting based on their family history, they may be treated differently during higher risk situations, like surgery or pregnancy. Women are also generally advised to avoid certain oral contraceptives, hormone replacement therapy, and other drugs that have estrogen if they have other good options. Generally, it is enough to know that a person has a risk based on their family history. The actual test results for a FVL or PT mutation may not change that treatment.

Although mutation test results usually don’t mean a person needs to take drugs to prevent clotting, some people find it useful to know for other reasons. For example, there are other ways to help prevent blood clots that don’t involve medications. Some people may be more conscious of preventing a clot or recognizing it early when they know they have a mutation.

Testing may not change how your doctor treats you.

There are many causes of serious pregnancy complications, like miscarriage, stillbirth, placental abruption, preeclampsia, and growth problems in the developing baby. An inherited tendency to clot (called thrombophilia) may be related, but studies haven’t agreed.

The factor V Leiden (FVL) and prothrombin (PT) G20210A mutations are two causes of thrombophilia. There are others. See Women and Thrombophilia for more information .

It isn't proven, but researchers suspect that thrombophilia may increase the chance of blood clots in the placenta (afterbirth). The placenta transfers oxygen and nutrients between the mother and developing baby. Clots may block the flow of these substances.

Many women have a FVL or PT mutation and don't have any pregnancy complications. This means a mutation alone does not explain why someone has a pregnancy loss or other complications.

There are drugs available that prevent clots, called blood thinners or anticoagulants. However, these drugs have risks. Experts don’t agree about whether anti-clotting treatment can lower the chance for pregnancy complications in women with a FVL or PT mutation. The risk from the drugs may be higher than the benefits. Studies are being done now to try to figure out if these drugs are useful for some women. Until then, most medical guidelines suggest no anti-clotting treatment for women with a mutation.

Because there is no agreed upon treatment for women with a mutation, and the risks for pregnancy complications are low overall, testing may not be useful.

Testing may not be useful for you.

You didn’t select any of the most common reasons people consider testing. The main signs of a factor V Leiden (FVL) or prothrombin (PT) mutation are discussed in the Who Should Consider Testing section. Even when people have signs that they might have a FVL or PT mutation, testing may still not make sense. The results usually don’t change how a person is treated.

Testing is not recommended for people without any sign that they may have a mutation. For example, even though these mutations raise the chance of clotting in women who take birth control pills, experts specifically don't recommend testing for all women who are considering or taking birth control pills.

If you have a reason for testing that isn't covered here, talk to your doctor or a genetic counselor about whether testing might be useful for you.