Venous Thromboembolism in Pregnancy

Venous thromboembolism in pregnancy encompasses deep vein thrombosis and pulmonary embolism occurring during gestation or the puerperium. The physiological hypercoagulability of pregnancy, together with venous stasis and vascular injury at delivery, raises the risk well above that of non-pregnant women of the same age, and pulmonary embolism remains an important cause of maternal death.

Definition

Venous thromboembolism in pregnancy is the formation of venous thrombi, manifesting as deep vein thrombosis or pulmonary embolism, occurring during pregnancy or the postpartum period in association with the prothrombotic physiological state of gestation.

Scope

This entry covers why pregnancy is a prothrombotic state, the timing and distribution of risk through pregnancy and the postpartum period, the population-level incidence, and the structure of guidance on prevention and treatment. It treats the topic as a clinical entity for reference and does not provide dosing or individualised management.

Core questions

• Why does normal pregnancy create a hypercoagulable, prothrombotic state?
• How is the risk of venous thromboembolism distributed across pregnancy and the postpartum period?
• How common is pregnancy-associated venous thromboembolism, and how has its measured incidence changed?
• On what evidence is thromboprophylaxis and treatment in pregnancy based?

Key concepts

• Virchow's triad in pregnancy
• Pregnancy hypercoagulability
• Venous stasis and venous compression
• Deep vein thrombosis
• Pulmonary embolism
• Postpartum risk peak
• Thromboprophylaxis with low-molecular-weight heparin

Mechanisms

Pregnancy fulfils all three components of Virchow's triad. The haemostatic system shifts toward a procoagulant state, with increases in several clotting factors and a reduction in natural anticoagulant activity and fibrinolysis, producing physiological hypercoagulability. Venous stasis develops as the gravid uterus compresses the pelvic and inferior caval veins and venous tone falls, and vascular injury occurs at delivery, particularly with caesarean section. The combination predisposes to deep vein thrombosis, which characteristically affects the left leg and iliofemoral veins, and to pulmonary embolism when thrombus embolises (Greer and Nelson-Piercy, 2005; American College of Obstetricians and Gynecologists, 2018).

Clinical relevance

Venous thromboembolism is a leading cause of maternal death in high-income countries, and its recognition and prevention are central to obstetric care, especially around delivery and the early postpartum weeks. This entry describes the predisposing physiology, risk distribution, and evidence framework as a reference; it is not a source of dosing or individualised treatment recommendations.

Epidemiology

Population-based data show that pregnancy and the puerperium substantially raise the risk of venous thromboembolism relative to non-pregnant women, with the highest absolute risk in the postpartum period; a 30-year population-based study documented this elevated and time-varying incidence (Heit and colleagues, 2005).

Evidence & guidelines

Because randomised trial evidence in pregnancy is limited, prophylaxis and treatment rest heavily on systematic reviews of low-molecular-weight heparin safety and efficacy and on consensus guidelines from haematology and obstetric bodies (Greer and Nelson-Piercy, 2005; Bates and colleagues, 2018; American College of Obstetricians and Gynecologists, 2018).

History

Recognition of pregnancy as a thrombotic risk state grew with twentieth-century understanding of coagulation and of Virchow's triad. The shift from unfractionated heparin and warfarin toward low-molecular-weight heparin, supported by systematic reviews of safety in pregnancy, reshaped prevention and treatment, and population-based studies clarified the timing of risk across pregnancy and the puerperium.

Seminal works

• heit-2005
• greer-2005
• bates-2018

Frequently asked questions

Why is pregnancy a risk factor for venous thromboembolism?

Pregnancy combines all three elements of Virchow's triad: a hypercoagulable shift in clotting, venous stasis from compression by the uterus and reduced venous tone, and vascular injury at delivery, which together raise the risk of clot formation.

When is the risk of clots highest in pregnancy?

Risk is elevated throughout pregnancy but is highest in the postpartum period, particularly the first weeks after delivery, which is why thromboprophylaxis decisions give special attention to that time.