Reducing the risk of venous thrombosis in pregnancy and after birth

A thrombosis is a blood clot in a blood vessel (a vein or an artery). Venous thrombosis occurs in a vein. Veins are the blood vessels that take blood back to the heart and lungs whereas arteries take the blood away.

A deep vein thrombosis (DVT) is a blood clot that forms in a deep vein of the leg, calf or pelvis.

Pregnancy increases your risk of a DVT, with the highest risk being just after you have had your baby.

However, venous thrombosis is still uncommon in pregnancy or in the first 6 weeks after birth, occurring in only 1–2 in 1000 women.

A DVT can occur at any time during your pregnancy, including the first 3 months, so it is important to see your midwife early in pregnancy.

Venous thrombosis can be serious because the blood clot may break off and travel in the bloodstream until it gets lodged in another part of the body, such as the lung. This is called a pulmonary embolism (PE) and can be life threatening. However, dying from a PE is very rare in women who are pregnant or who have just had a baby.

The symptoms of a PE can include:

• sudden unexplained difficulty in breathing
• tightness in the chest or chest pain
• coughing up blood (haemoptysis)
• feeling very unwell or collapsing

You should seek help immediately if you experience any of these symptoms. Diagnosing and treating a DVT reduces the risk of developing a PE.

Your risk of venous thrombosis is increased further if any of the following apply to you.

Before pregnancy

If you:

• are over 35 years of age
• have already had three or more babies
• have had a previous venous thrombosis
• have a mother, father, brother or sister who has had a venous thrombosis
• have a thrombophilia (a condition that makes a blood clot more likely)
• have a medical condition such as heart disease, lung disease or arthritis – your doctor or midwife will be able to tell you whether any medical condition you have increases your risk of a DVT/PE
• have severe varicose veins that are painful or above the knee with redness/swelling
• are a wheelchair user.

Lifestyle

If you:

• are overweight with a body mass index (BMI) over 30, are a smoker or if you use intravenous drugs.

During pregnancy

If you:

• are admitted to hospital
• are carrying more than one baby (multiple pregnancy)
• become dehydrated or less mobile in pregnancy due to, for example, vomiting in early pregnancy, being in hospital with a severe infection such as appendicitis or a kidney infection or if you are unwell from fertility treatment (ovarian hyperstimulation syndrome)
• are immobile for long periods of time, for example after an operation or when travelling for 4 hours or longer (by air, car or train)
• have pre-eclampsia – please see RCOG patient information Pre-eclampsia.

After the birth of your baby

If you:

• have a very long labour (more than 24 hours) or have had a caesarean section, lose a lot of blood after you have had your baby or receive a blood transfusion.

You may be able to reduce your risk, as most DVTs and PEs that occur during pregnancy and after birth are preventable.

You will have a risk assessment during pregnancy and after you have had your baby, during which your doctor or midwife will ask whether you have any of the risk factors above. This helps to decide whether you would benefit from preventive treatment. This will depend on which risk factors you have and how many.

Some risk factors, such as previous thrombosis, are significant enough on their own for treatment to be recommended. Other risk factors may not be enough on their own for you to require treatment. Your doctor or midwife will talk with you about your risk factors and explain why treatment may be advised in your case.

If you are diagnosed with a DVT, your doctor will give you treatment to reduce the risk of a PE occurring.

Before pregnancy

If you have any of the risk factors listed above and are planning a pregnancy you should talk to your GP or midwife. You may need to see an obstetrician early in pregnancy to discuss starting treatment.

If you have previously had a DVT or PE or have a thrombophilia (see above), your GP can arrange a hospital appointment with a doctor who specialises in thrombosis in pregnancy.

If you are already taking warfarin to treat or prevent venous thrombosis, you may be advised to change to heparin injections because warfarin can be harmful to your unborn baby (see section below). Most women are advised to change before becoming pregnant or as early as possible in pregnancy. For some women, warfarin may be the only option. Talk to your doctor before you become pregnant so that any changes can be planned to keep you and your baby as healthy as possible.

During and after pregnancy

Your midwife should carry out a risk assessment at your first antenatal booking and at around 28 weeks of pregnancy. A risk assessment should also be carried out if your situation changes during your pregnancy and/or if you are admitted to hospital. After your baby is born a further risk assessment should be done.

Yes. Your risk can either increase or decrease.

You may start by having one or two risk factors but your risk can increase if you develop other factors, such as becoming unwell, developing severe varicose veins, travelling for over 4 hours or having a complicated birth. In this case, you may be advised to start taking treatment.

Your risk may also decrease, for example if you stop smoking. Treatment may then no longer be necessary.

You can reduce your risk of getting of a DVT or PE:

• stay as active as you can
• wear special stockings (graduated elastic compression stockings) to help prevent blood clots
• keep hydrated by drinking normal amounts of fluids
• stop smoking
• lose weight before pregnancy if you are overweight.

You may be advised to start treatment with injections of heparin, which is an anticoagulant used to thin the blood. There are various types of heparin. The most commonly used in pregnancy is low-molecular- weight heparin (LMWH). Heparin is also used to treat venous thrombosis, but the dose of heparin used to prevent a venous thrombosis is usually less.

For most women, the benefits of heparin are that it reduces the risk of a venous thrombosis or a PE developing.

Heparin is given as an injection under the skin (subcutaneous) at the same time every day (sometimes twice daily). The dose is worked out for you depending on your risk factors and your weight in early pregnancy or before you became pregnant.

You may be on a low-dose or a high-dose regimen. You (or a family member) will be shown how and where in your body to give the injections. You will be provided with the needles and syringes (already made up) and will be given advice on how to store and dispose of these.

Low-molecular-weight heparin does not cross the placenta and therefore cannot harm your baby.

There may be some bruising where you inject – this will usually fade in a few days. One or two women in every 100 (1–2%) will have an allergic reaction. If you notice a rash after injecting, you should inform your doctor so that the type of heparin can be changed.

If you have any of the risk factors listed on page 2, you might need heparin during pregnancy. You should therefore see your GP, midwife or obstetrician as early as possible so that heparin can be started at the right time. For some women, this may be before their booking appointment.

The length of time you will be advised to stay on heparin depends on your risk factors and whether your situation changes. It may be that treatment is recommended for only a few days to cover long-distance travel, or treatment may be recommended for the week immediately after delivery. Sometimes, treatment may be recommended for the whole of your pregnancy and for up to 6 weeks after the birth.

If you think you are going into labour, do not have any more injections. Phone your maternity unit and tell them that you are on heparin treatment. They will advise you what to do.

An epidural injection (a regional anaesthetic injection given into the space around the nerves in your back to numb your lower body) cannot be given until 12 hours (24 hours if you are on a high dose) after your last injection. You will have the option of alternative pain relief.

If the plan is to induce labour, you should stop your injections 12 hours (24 hours if you are on a high dose) before the planned date.

If your baby needs to be born by emergency caesarean section within 12 hours (24 hours if you are on a high dose) of your last heparin injection you will not be able to have an epidural or spinal injection and instead will need a general anaesthetic for your operation.

If you are having a planned caesarean section, your last heparin injection should be 12 hours (24 hours if you are on a high dose) before the planned caesarean delivery. Heparin will usually be restarted within 4 hours of the operation.

It is important to be as mobile as possible after you have had your baby and to avoid becoming dehydrated.

A risk assessment will be carried out after the birth of your baby. Even if you weren’t having injections in pregnancy, you may need to start heparin injections for the first time after birth. This will depend on what risk factors you have for a DVT. You may be advised to have heparin for 7-10 days after birth or sometimes for 6 weeks after birth.

If you were on heparin before the baby’s birth, you are likely to be advised to continue this for 6 weeks afterwards.

If you were taking warfarin before pregnancy and have changed to heparin during pregnancy, you can change back to warfarin usually 3 days after birth.

At your postnatal appointment, your doctor should:

• discuss future pregnancies – you may be recommended heparin treatment during and after your next pregnancy but if, for example, you stop smoking or lose weight before your next pregnancy, heparin treatment may not be necessary next time
• discuss your options for contraception – you may be advised not to use any contraception that contains estrogen, such as the ‘combined pill’, as this can also add to your risk of DVT.

Yes – both heparin and warfarin are safe to take when breastfeeding.