Home / Patients / In-depth explanations
Pregnancy and risk of VTE: the use of anticoagulants during pregnancy

VTE and Pregnancy
A venous thromboembolism (VTE) is simply a blood clot in the vein. The vast majority occur in the leg veins. In pregnancy most of these clots are in the left leg, usually at the top of the leg or in the pelvis. It is clots in the veins of the leg deep within the muscles, not the veins under the skin, which cause most problems. This is called a deep venous thrombosis, or DVT for short. These DVTs are troublesome, but more worrying is when part of this clot breaks off and travels to the lung, where it can block part of the circulation. This is called a pulmonary embolism (PE). This can be life-threatening and is one of the most frequent causes of mothers dying during pregnancy in the developed world, albeit such deaths are very uncommon. It is important to prevent and treat DVT, as treatment can prevent this problem.

Clots are caused by sluggish blood flow, increased clotting tendency of the blood, and damage to the veins, although all three of these factors do not need to be present for a clot to occur. In pregnancy there is a marked reduction in blood flow through the leg veins. This reduced blood flow is clearly present from 16 weeks and is maximal at term. It takes around 6 weeks to return to normal after birth. The blood clotting system changes in pregnancy with increased levels of clotting factors in the blood. This is thought by doctors to be the body preparing for any blood loss during delivery. However, this also leads to an increased tendency during pregnancy for the blood to clot. Minor damage to the veins in the pelvis can easily occur at the time of delivery as the baby presses on these veins. Because of these changes, the risk of having a clot increases during pregnancy and after delivery. However, overall only 1 or 2 women in every 1000 will get a venous thrombosis during pregnancy. VTE will usually only occur when a women has several risk factors for VTE. Risk factors for developing a thrombosis are

• you have had a previous VTE
• you are very overweight
• you are over 35 years old
• you are immobile for long periods of time, for example after an operation
• you have pre-eclampsia
• you are recovering from a caesarean delivery
• you have another medical condition that predisposes you to thrombosis, such as active inflammatory bowel disease or some chronic kidney problems

Sometimes women are at greater risk of VTE because they have a particular tendency to clot. The body has natural systems to stop excessive clotting, but some people are born with a tendency for these systems to not work properly, giving them an increased risk of blood clots. This problem is called thrombophilia. Many deep venous thromboses in young women during pregnancy are the first sign of underlying thrombophilia. Often there is a family history of thrombosis, in which relatives such as mothers, fathers, aunts, and uncles have been affected. This is because this tendency can be passed down through the generations genetically. If you have had a clot or there is a family history of clotting, you will often be offered a blood test to determine if you have thrombophilia.

What are the signs of a DVT or PE?
Leg pain and marked swelling, especially in a woman with risk factors for thrombosis, is the way a blood clot in the leg usually comes to light. The diagnosis is usually confirmed by an ultrasound scan of the leg that shows a blood clot in the large vein at the top of the leg.

A pulmonary embolism often presents with sudden-onset chest pain, coughing up of blood, breathlessness, or even collapse. The tests that may be used to diagnose this condition include chest X-ray (which can also identify other problems that might be the cause of symptoms, such as a severe chest infection), CT X-ray scan of the lungs, or a VQ scan (ventilation perfusion) of the lungs. An ultrasound scan of the leg can also help, as often the clot will have started in the leg.

How is a VTE in pregnancy treated?
The treatment of DVT in pregnancy is similar to treatment when you are not pregnant. A medication called heparin is given. Heparin is an anticoagulant that 'thins the blood'. Heparin does not break down a clot, it simply prevents it from getting bigger. This allows your body time to gradually dissolve the clot. Heparin can either be injected under the skin or administered through the veins by a small pump, which carefully controls the rate of infusion. Blood tests may be required to check that you are getting the right dose. The dose often has to be adjusted to ensure that treatment is optimal. In non-pregnant women with a thrombosis, several days of heparin are followed by warfarin, which is a tablet that 'thins' the blood. However, warfarin freely crosses the placenta and can cause problems in the baby. If taken between 6 and 10 weeks of pregnancy it can occasionally cause abnormalities in the baby's development. In late pregnancy, it can cause bleeding problems. Where possible, doctors should avoid warfarin in pregnancy and continue with heparin injections, as heparin does not cross the placenta and the baby will not be exposed to this medication. It is safe to switch to warfarin following delivery. Warfarin appears safe for breast-feeding, as virtually none of this medication gets into breast milk. Special stockings (graduated elastic compression stockings), which can help reduce the swelling in the leg, and may also prevent long term damage to the veins in the leg should also be used. This long-term damage can result in leg discomfort, varicose veins, chronic swelling, and skin problems on the affected leg. This is call post-thrombotic syndrome. To prevent this complication the stockings need to be worn for up to two years after the clot occurs.

Does heparin have any side effects for the mother?
Heparin has some potential side effects for the mother in pregnancy, although as it does not cross the placenta, there is no direct risk to the developing baby. With use over several months, there is a small risk of heparin-induced osteoporosis or thinning of the bones. Occasionally allergies to heparin can occur.

Low-molecular-weight heparin for treatment of VTE in pregnancy
Recently newer forms of heparin called low-molecular-weight heparins have been used to treat VTE in pregnancy. These newer types of heparin appear to have much fewer side effects. They are highly effective at treating VTE, are easy to administer by self injection, and often need no blood test, as the dose is based on the patient’s weight. They do not appear to cross the placenta, and so have no effect on the baby. They are safe for breastfeeding. If you have had a DVT, you should discuss treatment with your doctor if you wish to find out more.

I have had a VTE before, how should I be treated?
If you have had a previous VTE, your risk of another will be increased during pregnancy. The risk is particularly increased if no cause was found for the DVT, if you have had more than one clot, or if you have thrombophilia. Most doctors believe that women with a previous clot should be tested for thrombophilia. This requires only a simple blood test. Ideally, this should be done before pregnancy. If you have had more than one DVT, doctors will usually recommend treatment with heparin or a low-molecular-weight heparin to prevent the recurrence of any clots. If you have had a previous clot and also suffer from thrombophilia then usually heparin or low–molecular-weight heparin is recommended. If you have had only one previous DVT with no underlying thrombophilia, then it is essential to weigh all the risk factors for your particular case, to determine if you need heparin treatment. This is a highly specialized area and specific advice about your particular situation must be obtained. Compression stockings may also be helpful in preventing thrombosis. It is not just women with a previous clot who require preventive treatment. Sometimes women with several risk factors will also be treated, especially after delivery, as the riskiest time for clots is in the few weeks after birth.

Does heparin have to be given by injection?
Heparin is not effective if given by mouth so it has to be injected. It is injected under the skin of the leg or the abdomen. Most women can be taught to inject themselves without difficulty.

After I give birth, can I take the oral contraceptive pill if I have had a VTE?
If you have had a VTE you should not usually take the combined (estrogen-containing) oral contraceptive pill because of the association between these estrogen-containing pills and venous thrombosis. You should use alternative forms of contraception. The progesterone-only pill, sometimes called the mini-pill, does not appear to be linked to any extra risk of thrombosis, so the 'mini-pill' can be used if you have had a previous VTE. You should, however, get specialist advice about contraception if you have had a VTE.