Does pregnancy raise the risk of venous clots? 

Pregnancy is a significant life event that involves numerous physiological changes, including those that affect how the blood circulates and clots. While the majority of pregnancies proceed without vascular complications, it is well-established that the risk of developing a venous clot is higher during this period compared to when a person is not pregnant. 

This article explains the biological reasons behind this increased risk, identifies the factors that can further elevate it, and provides clear guidance on identifying symptoms that require medical attention. By understanding these risks, expectant parents can take proactive steps to support their circulatory health throughout their pregnancy and in the weeks following birth. 

What We’ll Discuss in This Article 

• The statistical increase in clot risk during pregnancy and the postnatal period. 

• How hormonal changes and blood flow alterations contribute to clotting. 

• The physical impact of the growing uterus on pelvic veins. 

• Common triggers that can lead to a venous thromboembolism (VTE). 

• How to distinguish between normal pregnancy swelling and signs of a clot. 

• Practical steps for prevention and when to seek emergency help. 

Why Pregnancy Increases the Risk of Venous Blood Clots 

Yes, pregnancy increases the risk of venous clots, such as Deep Vein Thrombosis (DVT), by approximately four to six times compared to those who are not pregnant. This elevated risk is present from the first trimester, increases as the pregnancy progresses, and is highest in the first six weeks following childbirth. This is primarily due to natural changes designed to protect the body during delivery. 

The body naturally enters a ‘hypercoagulable’ state during pregnancy, meaning the blood clots more easily. While this serves as a protective mechanism to prevent excessive bleeding during labour and birth, it also makes the formation of unnecessary clots in the veins more likely. 

• Prevalence: Although the risk is higher, clots remain relatively uncommon, affecting roughly 1 to 2 in every 1,000 pregnant individuals. 

• Timing: A clot can occur at any stage, but the postpartum period (after birth) is the most critical window for monitoring. 

• Location: Most pregnancy-related clots form in the deep veins of the legs or the pelvis. 

Clinical context shows that healthcare providers conduct VTE risk assessments at the start of pregnancy, during any hospital stays, and after delivery to determine if preventative measures, such as blood-thinning injections, are necessary. 

The Physiology of Clotting in Pregnancy 

The increased risk of clots during pregnancy is caused by a combination of ‘sticky’ blood, slower blood flow, and potential damage to blood vessels. These three factors, known as Virchow’s Triad, are all influenced by the biological shifts of pregnancy. Changes in protein levels in the blood and the physical pressure of the baby on the veins create a unique environment for thrombus formation. 

During pregnancy, the body produces more clotting factors (proteins that help blood clot) and fewer anticoagulants (proteins that prevent clots). This shift ensures that the body can quickly form a ‘plug’ once the placenta detaches. 

Key physiological shifts include: 

• Venous Stasis: The hormone progesterone causes the walls of the veins to relax and widen, which can lead to blood ‘pooling’ in the legs. 

• Mechanical Obstruction: As the uterus grows, it can physically press on the large veins in the pelvis, slowing the return of blood from the legs to the heart. 

• Vessel Changes: The stretching and pressure on pelvic veins during both pregnancy and delivery can cause microscopic damage to the vessel walls, which the body may respond to by forming a clot. 

Causes of Increased Clot Risk 

Beyond the natural state of pregnancy, several specific causes can lead to a significant rise in the likelihood of a venous event. These are often related to a person’s medical history or the specific circumstances of their pregnancy. 

• Previous Medical History: A personal or family history of blood clots or known blood-clotting disorders (thrombophilias). 

• Maternal Age and BMI: Being over the age of 35 or having a Body Mass Index (BMI) of 30 or higher. 

• Multiple Pregnancy: Carrying twins, triplets, or more increases the physiological load on the circulatory system. 

• Underlying Health Conditions: Conditions such as heart disease, lung disease, or certain inflammatory disorders like lupus. 

• Assisted Conception: Pregnancies resulting from certain fertility treatments, such as IVF, may carry a slightly higher baseline risk. 

Triggers for Venous Clots in Pregnancy 

Triggers are external factors or temporary events that can provoke the formation of a clot in someone who is already at a higher risk due to pregnancy. 

• Immobility: Long periods of bed rest due to pregnancy complications or recovery after surgery (such as a Caesarean section). 

• Dehydration: Severe morning sickness (hyperemesis gravidarum) can lead to fluid loss, making the blood more concentrated. 

• Long-Distance Travel: Sitting still for more than four hours, whether by car, train, or plane, significantly slows circulation. 

• Infection: Serious systemic infections or those requiring hospitalisation can trigger the body’s inflammatory and clotting responses. 

• Surgical Intervention: Any major surgery during pregnancy or shortly after birth, including an emergency C-section, acts as a physical trigger. 

Differentiation: Normal Pregnancy Swelling vs. DVT 

It is common for pregnant individuals to experience swelling (oedema) in both legs, especially in the third trimester. However, it is vital to know when this swelling indicates a potential medical emergency. 

Feature	Normal Pregnancy Swelling	Deep Vein Thrombosis (DVT)
Location	Usually affects both legs and ankles equally.	Usually affects only one leg (often the left).
Pain Level	General heaviness or mild discomfort.	Significant pain, tenderness, or cramping.
Skin Appearance	Skin colour remains normal or slightly pale.	Skin may look red, blue, or purple.
Temperature	Legs feel normal or cool to the touch.	The swollen area feels warm or hot.
Progression	Often improves with rest or elevating the feet.	Pain and swelling persist or worsen despite rest.

If you experience sudden shortness of breath or chest pain along with leg swelling, this may indicate that a clot has moved to the lungs (Pulmonary Embolism). 

Conclusion 

Pregnancy and the weeks following birth are periods of naturally increased risk for venous clots due to changes in blood composition and circulation. While most people will not experience a clot, being aware of your individual risk factors and the specific triggers ‘ such as immobility or dehydration ‘ is essential. Staying active, drinking plenty of fluids, and attending all antenatal appointments for risk screening are the best ways to manage this risk. 

If you experience severe, sudden, or worsening symptoms, call 999 immediately. 

You may find our free BMI Calculator helpful for understanding how your weight might influence your vascular risk profile during your pregnancy journey. 

Authority Snapshot (E-E-A-T Block) 

This article was reviewed by Dr. Rebecca Fernandez, a UK-trained physician with extensive clinical experience in general surgery, cardiology, and internal medicine. Dr. Fernandez has managed high-risk patients in intensive care and emergency settings, ensuring that the information provided is medically accurate and aligned with NHS standards. This guide provides evidence-based safety information regarding the vascular risks associated with pregnancy and the postnatal period.