Venous thromboembolism in pregnancy and after birth: risk, prevention and treatment

Venous thromboembolism, or VTE, covers a clot in a deep vein, usually of the leg, calf or pelvis (deep vein thrombosis, DVT), and a clot that travels to the lungs (pulmonary embolism, PE). Pregnancy changes blood clotting so that bleeding at birth is easier to control, and the side effect is that clots form more readily. The RCOG puts the overall chance of venous thrombosis at only 1 to 2 in 1,000 women, with the risk highest straight after delivery.

Some risk factors exist before pregnancy and others arise during or after it. A midwife or doctor weighs them together.

• Before pregnancy: age over 35, three or more previous pregnancies, previous clots or a family history, an inherited tendency (thrombophilia), medical conditions such as heart or lung disease or arthritis, severe varicose veins, using a wheelchair, a BMI over 30, smoking or injecting drug use.

• During or after pregnancy: hospital admission, multiple pregnancy, dehydration, immobility for more than 4 hours, pre-eclampsia, labour lasting more than 24 hours, a caesarean birth or heavy blood loss.

Your risk score is reviewed at the first antenatal booking, at around 28 weeks, whenever you are admitted to hospital and again after the birth. Depending on the result you may be advised to keep moving and drink plenty of fluids, wear graduated compression stockings, or take heparin injections for part or all of pregnancy and for some days or up to 6 weeks afterwards.

Long journeys deserve thought. Sitting still for more than 4 hours raises the risk, so your midwife may suggest moving regularly, and a short course of heparin if you have other risk factors.

Low-molecular-weight heparin is a blood thinner given by injection under the skin, once or twice a day. The dose depends on your weight in early pregnancy. You or your partner can be taught to give it at home. According to the RCOG, it does not cross the placenta to reach the baby, and it is safe when breastfeeding.

Keep your maternity team informed about bruising at injection sites, bleeding, or any allergic reaction. When labour begins, stop injections and call the maternity unit. An epidural cannot be placed until 24 hours after the last heparin injection, and for a planned caesarean injections are stopped 24 hours before the operation and restarted within 4 hours afterwards.

DVT usually affects one leg. You might notice a red, hot, swollen leg, swelling of part or all of the leg, heaviness, or pain and tenderness that you feel mostly when standing or walking. Pulmonary embolism may cause sudden difficulty breathing, chest tightness or pain, coughing up blood, or collapse. Treat these as emergencies and call 999, since a pulmonary embolism can be life-threatening.

For a suspected DVT, an ultrasound scan of the leg is used and may be repeated if the first is normal but symptoms continue. For a suspected PE, tests can include a chest X-ray, a CT scan or a ventilation-perfusion (VQ) scan, sometimes alongside leg ultrasound. The RCOG explains that a chest X-ray uses a tiny dose of radiation not considered harmful, and that CT and VQ scans carry small risks which are outweighed by the importance of diagnosing a PE.

Treatment is with LMWH for the rest of the pregnancy and for at least 6 weeks after the birth, with a minimum total of 3 months. After the birth you can choose between continuing heparin and switching to warfarin tablets, and both are safe with breastfeeding.