Introduction

Thrombosis, the pathological formation of a blood clot within a blood vessel, poses a significant threat to maternal health during pregnancy and the postpartum period. During pregnancy, the body naturally becomes more prone to clotting to prevent excessive bleeding during childbirth.¹ However, this same mechanism significantly increases the risk of venous thromboembolism (VTE), including deep vein thrombosis (DVT) and pulmonary embolism (PE). VTE remains one of the leading causes of maternal morbidity and mortality globally.² In particular, PE has been identified as a primary cause of maternal death in many developed countries.³

This article delves into the complex interplay between thrombosis and pregnancy, highlighting risk factors, underlying pathophysiology, potential complications, and the approaches to prevention, diagnosis, and management.

Physiological Changes in Pregnancy Favoring Thrombosis

During pregnancy, the body goes through major blood and vessel changes to prevent excessive bleeding during childbirth. Factors increasing the likelihood of blood clotting include:

• Increased clotting factors (like fibrinogen and factors VII, VIII, IX, and X) make the blood more viscous and more prone to clotting
• Decrease in natural anticoagulants (like protein S) and increased resistance to protein C further promote clotting
• The growing uterus presses on major veins (like the inferior vena cava), slowing blood flow in the legs, which can lead to clots
• Delivery itself can cause blood vessel injury, adding to the risk

All these factors create a higher chance of developing blood clots during pregnancy and after delivery.⁴ 

Incidence and Epidemiology

Blood clots, known as VTE, are a significant concern during pregnancy. They occur in about 1 to 2 pregnancies out of every 1,000, which is 4 to 5 times higher than in women of the same age who are not pregnant. This increased risk is mainly due to the body’s natural changes during pregnancy that make blood more likely to clot, helping to prevent bleeding during childbirth.⁵ 

The risk varies throughout the course of pregnancy. It is present during all three trimesters but becomes much higher after delivery especially during the first 6 weeks postpartum when the body is recovering and blood flow may still be slower. Among pregnancy-related blood clots, DVT is more common.⁶ Symptoms include leg pain, swelling (usually in one leg), redness, and warmth. However, the more dangerous complication is PE. This occurs when a blood clot, often originating in the leg, travels to the lungs and obstructs blood flow. PE is less common than DVT but far more life-threatening, causing chest pain, shortness of breath, rapid heartbeat, and, in severe cases, collapse or death.

Types of Thrombosis in Pregnancy

During pregnancy, the two most common types of thrombosis are DVT and PE. DVT happens when a blood clot forms in a deep vein, usually in the leg. This can cause pain, swelling, warmth, and redness often in one calf. If the clot breaks loose and travels to the lungs, it can lead to a PE, which is life-threatening. PE symptoms include sudden shortness of breath, chest pain, and coughing up blood, and it requires immediate emergency care.⁷ 

Pregnancy naturally increases the risk of thrombosis due to hormonal changes, slower blood flow from the growing uterus pressing on veins, and the body’s preparation for delivery. Some factors raise this risk even more including a personal or family history of clots, being over 35 years old, obesity, carrying twins or more, having preeclampsia, or having a C-section. Smoking, severe varicose veins, dehydration, and long periods of immobility (like long-haul travel) can also contribute to the risk.

If DVT develops, treatment usually involves heparin injections, a blood-thinner that prevents the clot from growing and reduces the chance of PE. Heparin is safe for both the mother and baby, and treatment typically continues throughout pregnancy and for at least six weeks postpartum. To help prevent clots, staying active, wearing compression stockings, staying hydrated, and moving frequently during travel are all important steps. Early diagnosis and management are key to reducing complications and ensuring a safer pregnancy.⁸ 

Risk Factors for Thrombosis in Pregnancy

Prolonged immobility, such as long periods of sitting during travel or bed rest due to pregnancy complications, slows blood flow in the legs, increasing the chance of clot formation. Surgery, particularly cesarean sections, and injuries to blood vessels also contribute to higher risk. Pregnancy itself puts added pressure on the veins in the pelvis and legs, and this risk remains for up to six weeks after delivery. Hormonal medications like birth control or hormone replacement therapy further increase clotting tendency.

Additional risk factors include obesity which increases pressure on the veins,and smoking which affects blood flow and clotting, raising the risk. Cancer, certain cancer treatments, and chronic illnesses like heart failure or inflammatory bowel diseases (Crohn’s disease or ulcerative colitis) can promote clot formation. A personal or family history of DVT or PE also increases susceptibility. Additionally, genetic factors, such as Factor V Leiden mutation or other inherited clotting disorders, make the blood more prone to clotting — especially when combined with other risk factors. Understanding these risks helps in identifying high-risk pregnancies and ensuring timely prevention and treatment.⁹ 

Complications of Thrombosis in Pregnancy 

Thrombosis during pregnancy can lead to serious complications for both the mother and the baby. For the mother, one of the most common long-term issues is post-thrombotic syndrome (PTS) , marked by chronic leg pain, swelling, and skin changes due to poor blood flow.¹⁰ There’s also a heightened risk of recurrent VTE, which increases with each future pregnancy if not properly managed. A more severe complication, chronic thromboembolic pulmonary hypertension (CTEPH), can occur if PE remains unresolved, causing persistent high blood pressure in the lungs.¹¹ Additionally, the use of anticoagulants essential for preventing further clots — carries a risk of bleeding, especially around delivery.

For the baby, maternal thrombosis can restrict blood flow to the placenta, leading to intrauterine growth restriction (IUGR), where the baby doesn’t grow as expected. It can also trigger preterm labor or placental abruption, a serious condition where the placenta detaches from the uterus prematurely. In severe cases, particularly involving antiphospholipid syndrome (APS), the risk of stillbirth increases, especially in the second or third trimester. These risks highlight the importance of early detection, careful monitoring, and proactive management to safeguard maternal and fetal health.¹² 

Diagnosing DVT and PE in Pregnancy

Pregnant women have a higher risk of DVT, especially in the left leg, which can lead to PE, a serious potentially life-threatening condition. Diagnosis is difficult because symptoms like leg swelling and discomfort are common in pregnancy.

DVT Diagnosis:

• Wells' criteria is not reliable in pregnancy; the LEFt rule (left-leg symptoms, calf difference >2 cm, first trimester) is more effective
• D-dimer tests are less helpful due to natural increases, but newer high-sensitivity methods show promise
• Compression ultrasound (CUS) is the first-line imaging test. If results are unclear, serial ultrasounds or magnetic resonance venography (MRV) are safer alternatives to CT venography (which involves radiation)^13,14  

PE Diagnosis:

• Symptoms like chest pain, shortness of breath, and elevated heart rate are common in pregnancy, making diagnosis tricky
• The PERC rule doesn’t apply to pregnancy. The pregnancy-adapted YEARS model and Pregnancy-Adapted Geneva (PAG) Score combine symptoms with D-dimer to guide decisions, but more validation is needed
• Chest X-ray is done first to rule out other causes. For definitive imaging, doctors choose between:
  • CT pulmonary angiography (CTPA) higher radiation to the mother’s chest
  • Ventilation-perfusion (V/Q) scans slightly more fetal exposure but avoids contrast dye
  • MRI-based options are emerging for safer, radiation-free diagnosis¹⁵ 

Conclusion 

Thrombosis during pregnancy, especially in the form of DVT and PE, represents a critical threat to maternal and fetal health. The physiological adaptations that prepare a woman’s body for childbirth such as increased clotting factors and reduced anticoagulant activity create a hypercoagulable state that, while protective against bleeding, significantly elevates the risk of VTE. This risk is further compounded by factors like advanced maternal age, immobility, obesity, multiple pregnancies, and underlying thrombophilia. 

DVT is more common than PE, but PE carries a higher risk of fatality, making early identification and intervention essential. Diagnosis remains challenging due to symptom overlap with normal pregnancy and limitations of conventional tools. However, advancements like the LEFt rule, pregnancy-adapted YEARS algorithm, and safer imaging techniques are improving clinical outcomes. 

Proper management with anticoagulants, mainly low molecular weight heparin, has proven effective in both treatment and prevention. Awareness, risk stratification, and a multidisciplinary approach to care are vital. By prioritising early detection and evidence-based management, healthcare providers can significantly reduce the burden of thrombosis during pregnancy and ensure better safety for both mother and baby.