Venous Thromboembolism in Pregnancy

Venous thromboembolism (VTE) is a common and potentially fatal condition. It involves blood clots (thrombosis) developing in the circulation. Thrombosis occurs as a result of stagnation of blood, and in hyper-coagulable states, such as pregnancy.

When a thrombosis develops in the venous circulation, it is called deep vein thrombosis (DVT). This thrombosis can mobilise (embolisation) from the deep veins and travel to the lungs, where it becomes lodged in the pulmonary arteries. This blocks blood flow to related areas of the lungs, and is called a pulmonary embolism (PE).

Pulmonary embolism is a significant cause of death in obstetrics. The risk is significantly reduced with VTE prophylaxis. The risk is highest in the postpartum period.


Risk Factors

There is a long list of risk factors for VTE in pregnancy:

  • Smoking
  • Parity ≥ 3
  • Age > 35 years
  • BMI > 30
  • Reduced mobility
  • Multiple pregnancy
  • Pre-eclampsia
  • Gross varicose veins
  • Immobility
  • Family history of VTE
  • Thrombophilia
  • IVF pregnancy


The RCOG guidelines (2015) advise starting prophylaxis from:

  • 28 weeks if there are three risk factors
  • First trimester if there are four or more of these risk factors


There are additional scenarios where prophylaxis is considered, even in the absence of other risk factors:

  • Hospital admission
  • Surgical procedures
  • Previous VTE
  • Medical conditions such as cancer or arthritis
  • High-risk thrombophilias
  • Ovarian hyperstimulation syndrome


VTE Prophylaxis

All pregnant women should have a risk assessment for their risk of venous thromboembolism (VTE) at booking. A risk assessment is performed again after birth. Additional risk assessments are necessary at other times, such as if they are admitted to hospital, undergo a procedure or develop significant immobility Each unit will have a policy and protocol for assessing risk and starting prophylaxis in pregnancy.

Women at increased risk of VTE should receive prophylaxis with low molecular weight heparin (LMWH) unless contraindicated. Examples of LMWH are enoxaparindalteparin and tinzaparin. Prophylaxis is started as soon as possible in very high risk patients and at 28 weeks in those at high risk. It is continued throughout the antenatal period and for six weeks postnatally.

Prophylaxis is temporarily stopped when the woman goes into labour, and can be started immediately after delivery (except with postpartum haemorrhage, spinal anaesthesia and epidurals).

Mechanical prophylaxis may be considered in women with contraindications to LMWH. The options for mechanical prophylaxis are:

  • Intermittent pneumatic compression with equipment that inflates and deflates to massage the legs
  • Anti-embolic compression stockings



Deep vein thrombosis is almost always unilateral. Bilateral DVTs are rare, and bilateral symptoms are more likely due to an alternative diagnosis such as chronic venous insufficiency or heart failure. DVTs present with:

  • Calf or leg swelling
  • Dilated superficial veins
  • Tenderness to the calf (particularly over the deep veins)
  • Oedema
  • Colour changes to the leg


To examine for leg swelling measure the circumference of the calf 10cm below the tibial tuberosityMore than 3cm difference between calves is significant.


Pulmonary embolism can present with subtle signs and symptoms. In patients with potential features of a PE, risk factors for PE, and no other explanation for their symptoms, have a low threshold for suspecting a PE. Presenting features include:

  • Shortness of breath
  • Cough with or without blood (haemoptysis)
  • Pleuritic chest pain
  • Hypoxia
  • Tachycardia (this can be difficult to distinguish from the normal physiological changes in pregnancy)
  • Raised respiratory rate
  • Low-grade fever
  • Haemodynamic instability causing hypotension



Doppler ultrasound is the investigation of choice for patients with suspected deep vein thrombosis. The RCOG guideline (2015) recommends repeating negative ultrasound scans on day 3 and 7 in patients with a high index of suspicion for DVT.

Women with suspected pulmonary embolism require:

  • Chest xray
  • ECG


There are two main options for establishing a definitive diagnosis: CT pulmonary angiogram (CTPA) or ventilation-perfusion (VQ) scan.

CT pulmonary angiogram involves a chest CT scan with an intravenous contrast that highlights the pulmonary arteries to demonstrate any blood clots. This is usually the first choice for investigating a pulmonary embolism, as it tends to be more readily available, provides a more definitive assessment and gives information about alternative diagnoses such as pneumonia or malignancy.

Ventilation-perfusion (VQscan involves using radioactive isotopes and a gamma camera, to compare the ventilation with the perfusion of the lungs. First, the isotopes are inhaled to fill the lungs, and a picture is taken to demonstrate ventilation. Next, a contrast containing isotopes is injected, and a picture is taken to demonstrate perfusion. The two images are compared. With a pulmonary embolism, there will be a deficit in perfusion, as the thrombus blocks blood flow to the lung tissue. This area of lung tissue will be ventilated but not perfused.

When considering the choice between CTPA and VQ scan:

  • CTPA is the test for choice for patients with an abnormal chest xray
  • CTPA carries a higher risk of breast cancer for the mother (minimal absolute risk)
  • VQ scan carriers a higher risk of childhood cancer for the fetus (minimal absolute risk)


Patients with a suspected deep vein thrombosis and pulmonary embolism should have a Doppler ultrasound initially, and if a DVT is present, they do not require a VQ scan or CTPA to confirm a PE. The treatment for DVT and PE are the same.

TOM TIP: The Wells score is not validated for use in pregnant women. D-dimers are not helpful in pregnant patients, as pregnancy is a cause of a raised D-dimer. 



Management of venous thromboembolism in pregnancy is with low molecular weight heparin (LMWH). Examples of LMWH are enoxaparindalteparin and tinzaparin. The dose is based on the woman’s weight at the booking clinic, or from early pregnancy.

LMWH should be started immediately, before confirming the diagnosis in patients where DVT or PE is suspected and there is a delay in getting the scan. Treatment can be stopped when the investigations exclude the diagnosis.

When the diagnosis is confirmed, LMWH is continued for the remained of pregnancy, plus six weeks postnatally, or three months in total (whichever is longer). There is an option to switch to oral anticoagulation (e.g. warfarin or a DOAC) after delivery. An individual risk assessment is performed before stopping anticoagulation, with advice from a haematologist if necessary.

Women with a massive PE and haemodynamic compromise need immediate management by an experienced team of medical doctors, obstetricians, radiologists and others. This is a life-threatening scenario. Treatment options are:

  • Unfractionated heparin
  • Thrombolysis
  • Surgical embolectomy


Last updated September 2020