Pregnancy and Blood Clots: Risk, Warning Signs, and Prevention

Dec 7, 2019
12 min read

Updated: Nov 5

Blood clots are a leading yet preventable cause of complications during pregnancy and after childbirth. Rising hormone levels, slowed circulation, and physical changes all contribute to a higher risk of clotting, especially in the weeks following delivery. Recognizing symptoms early and following medical guidance on prevention, such as movement, hydration, and prescribed anticoagulants, can safeguard both mother and child.

Overview

Why does pregnancy increase the risk of developing blood clots?

Pregnancy and the postpartum period represent one of the most physiologically high-risk times for venous thromboembolism (VTE), a condition that includes deep vein thrombosis (DVT) and pulmonary embolism (PE). While blood clots can affect anyone, the hormonal, vascular, and mechanical changes of pregnancy make clot formation five times more likely than in nonpregnant women, according to the Centers for Disease Control and Prevention (CDC) and studies published in The New England Journal of Medicine (2018). The encouraging news: with proper awareness and medical care, most pregnancy-related clots are preventable and treatable.

Mechanism

What physiological changes during pregnancy raise clotting potential?

Pregnancy initiates one of the most complex hemostatic transformations in human physiology. The maternal body recalibrates its circulatory and coagulation systems to prevent hemorrhage during delivery, but these same adaptations substantially increase the potential for clot formation. This heightened coagulation state—driven by a combination of hormonal, mechanical, and vascular changes—forms the foundation for pregnancy-associated thrombosis.

Hormonal and Hematologic Adaptations

Estrogen and progesterone levels rise sharply during pregnancy, increasing hepatic synthesis of fibrinogen and clotting factors VII, VIII, IX, and X while simultaneously reducing anticoagulant proteins such as protein S. This results in a state of hypercoagulability, in which clot formation is easier and clot breakdown is slower. At the same time, fibrinolytic activity declines through elevated plasminogen activator inhibitors (PAI-1 and PAI-2), reducing the body’s ability to dissolve existing clots.

This physiologic recalibration is protective at delivery but introduces persistent clotting risk from early pregnancy through the postpartum period. The American Journal of Obstetrics & Gynecology (2023) reports that this hypercoagulable state increases venous thromboembolism (VTE) risk by four to fivefold during pregnancy and by up to twentyfold after birth.

Mechanical and Circulatory Changes

As the uterus enlarges, it exerts progressive pressure on the inferior vena cava and pelvic veins, particularly in the third trimester. This compression slows venous return from the lower extremities, causing venous stasis, a key component of Virchow’s triad for thrombosis. Doppler studies published in Thrombosis and Haemostasis (2021) show that venous flow velocity in the femoral veins can decrease by as much as 50% in late pregnancy. Prolonged standing, dehydration, or immobility intensifies this stagnation, especially in women with pre-existing venous weakness or varicosities.

Vascular and Endothelial Dysfunction

Endothelial cells lining the blood vessels undergo inflammatory and structural changes under the combined influence of oxidative stress, placental signaling molecules, and hormonal shifts. In uncomplicated pregnancies, this mild dysfunction remains compensatory. However, when compounded by pathologic conditions such as preeclampsia or eclampsia, the damage becomes systemic.

Preeclampsia is characterized by hypertension, endothelial injury, and platelet activation, all of which accelerate clot formation and impair vascular reactivity. Eclampsia represents the severe end of this spectrum, where widespread endothelial disruption leads to seizures, multiorgan strain, and a sharp increase in thrombotic potential. The BMJ (2022) notes that women with preeclampsia experience a threefold greater incidence of deep vein thrombosis (DVT) or pulmonary embolism (PE), often extending into the postpartum phase.

Fluid Retention and Edema

During pregnancy, plasma volume expands by 30–50%, increasing hydrostatic pressure and venous distension. This physiological edema, particularly in the lower limbs, impairs microcirculatory flow and can exacerbate venous pooling. In patients with chronic venous insufficiency (CVI) or existing varicosities, these pressures further weaken venous valves, allowing blood to reflux and stagnate. The resultant inflammatory cascade not only worsens swelling and discomfort but also fosters local thrombus formation.

Interaction of Systemic and Local Factors

The interplay between hypercoagulability, mechanical obstruction, endothelial injury, and venous dilation creates a perfect storm for clot formation. When these mechanisms coincide with additional triggers—such as dehydration, infection, gestational diabetes, or cesarean recovery—the risk of venous thromboembolism (VTE) multiplies.

In essence, the mechanism of pregnancy-related thrombosis is not the product of a single cause but the convergence of hormonal amplification, mechanical compression, and vascular compromise. These processes are physiologic in intent but pathologic in effect, transforming the body’s natural preparation for childbirth into one of the most thrombosis-prone states in medicine.

Risk Factors

Which medical, genetic, or lifestyle factors heighten the likelihood of clot formation in expectant mothers?

A number of individual, genetic, and medical factors can significantly amplify the risk of venous thromboembolism (VTE) during pregnancy and the postpartum period. Each influences circulation, hormone balance, or vascular integrity, thereby exacerbating the body’s natural hypercoagulable state.

Previous Blood Clot

Prior deep vein thrombosis (DVT) or pulmonary embolism (PE) is the most powerful predictor of recurrence. Without preventive anticoagulation, recurrence risk during a subsequent pregnancy can exceed 10%, according to data from The American Journal of Obstetrics & Gynecology (2021). Prophylactic use of low-molecular-weight heparin (LMWH) has been shown to dramatically reduce this risk and is now standard for women with a history of clotting.

Inherited Thrombophilias

Genetic conditions such as Factor V Leiden, prothrombin G20210A mutation, and antithrombin deficiency increase clot risk three to tenfold, per Blood (2020). These mutations disrupt normal clot regulation and increase the risk of thrombus formation, especially when combined with pregnancy-related hormonal changes.

Family History of VTE

Having a first-degree relative with a history of blood clots doubles the likelihood of developing one. This link reflects both shared genetic predispositions and environmental exposures, such as hormonal therapy or immobility during pregnancy.

Cesarean Delivery (C-Section)

Women who undergo cesarean delivery face roughly twice the risk of venous thromboembolism (VTE) compared to those with vaginal births. The combination of surgical vessel injury, anesthesia-related immobility, and postoperative inflammation activates coagulation pathways and slows venous return.

Prolonged Immobility

Extended bed rest, whether prescribed for pregnancy complications or required during recovery, reduces muscle movement that normally propels blood through deep veins. This lack of circulation fosters venous stasis, one of the primary drivers of clot formation.

Pregnancy Complications

Conditions such as preeclampsia, placental abruption, and postpartum hemorrhage further increase clotting activity. These complications promote endothelial injury and systemic inflammation, amplifying the body’s natural coagulation response.

Chronic Medical Conditions

Underlying diseases like heart failure, chronic lung disease, or diabetes elevate baseline inflammation and impair vascular function. These conditions heighten the risk of both arterial and venous thrombosis during pregnancy.

Maternal Age 35 Years or Older

Advancing maternal age is associated with decreased vascular elasticity, slower venous return, and a higher incidence of coexisting conditions such as hypertension or obesity, all of which contribute to thrombosis risk.

Overweight or Obesity

Excess weight increases abdominal and pelvic pressure, impeding venous drainage from the lower limbs. Research in The Lancet Haematology (2021) shows that obesity can triple the risk of venous thromboembolism (VTE) in late pregnancy and postpartum, particularly when combined with cesarean delivery or inactivity.

Smoking

Tobacco use damages the endothelium and accelerates platelet aggregation, compounding the clotting effects of pregnancy hormones. Smoking during pregnancy also magnifies vascular inflammation and reduces oxygen delivery, further destabilizing circulatory health.

Pregnancy-related venous thromboembolism (VTE) most often develops in the legs or pelvis and may occur up to three months postpartum, a period identified as the window of highest risk by The Lancet Haematology (2022). Recognizing early warning signs, such as leg pain, swelling, redness, or sudden shortness of breath, remains essential for timely intervention and prevention of life-threatening complications.

Prevention

How can I protect myself and my unborn child from possible pregnancy complications?

Pregnancy is a physiologic hypercoagulable state designed to protect against hemorrhage at delivery, yet this same adaptation can create a dangerous environment for placental thrombosis and fetal hypoxia when clotting mechanisms become overactive. According to the Journal of Thrombosis and Haemostasis (2023), coagulation factor activity—particularly fibrinogen, factor VII, factor VIII, and von Willebrand factor—increases up to fivefold during normal gestation, while protein S, one of the body’s key natural anticoagulants, declines by as much as 60%. This shift is protective in controlled balance, but when compounded by additional prothrombotic tendencies—whether genetic, hormonal, or inflammatory—it can lead to microvascular occlusion within the placenta and compromise fetal oxygenation.

Importantly, this imbalance is not confined to women with a known history of clots or diagnosed thrombophilia. Data from Blood Advances (2024) indicate that nearly half of thrombotic pregnancy complications occur in women with no prior clotting history or identifiable disorder, underscoring that the physiologic hypercoagulability of pregnancy alone can unmask previously silent risk. Even in low-risk pregnancies, second or third-trimester bleeding accompanied by visible clots should prompt further investigation rather than basic reassurance.

Fetal Considerations

How can the natural increase in blood clotting during pregnancy affect my baby?

When maternal clotting exceeds physiologic limits, microthrombi can form in the uteroplacental circulation, obstructing oxygen and nutrient transfer to the fetus. The American Journal of Obstetrics & Gynecology (2023) reports that 30–50% of stillbirths show placental infarction or intervillous thrombosis on histopathology, often in women without overt maternal symptoms. Similarly, Placenta Health Consortium (2024) data indicate that over 40% of cases of intrauterine growth restriction (IUGR) involve placental fibrin deposition, consistent with thrombotic obstruction.

This same pathophysiology explains the well-documented associations between hypercoagulability and miscarriage, preeclampsia, and placental abruption. A meta-analysis in Obstetrics & Gynecology (2023) found that women with unrecognized thrombophilia were 2.6 times more likely to experience early miscarriage and 3.8 times more likely to have recurrent pregnancy loss. Likewise, a population study in BMJ Open (2022) identified a threefold increase in preeclampsia risk among women with elevated fibrinogen or D-dimer levels, both of which are markers of excessive coagulation activity.

Even mild or intermittent bleeding with clots later in pregnancy is cause for concern. In a BJOG (2023) cohort, second-trimester bleeding with clot passage was associated with a fivefold higher risk of placental abruption and double the rate of preterm delivery, suggesting that subclinical thrombosis may precede overt placental failure.

Traditionally, obstetric protocols have limited thrombophilia screening to women with personal or family histories of clotting. However, this approach fails to account for the temporary but clinically significant prothrombotic phenotype that pregnancy itself induces. A prospective study in Haematologica (2024) found that 48% of women who developed significant ischemic placental disease or thrombotic complications during pregnancy had no prior clotting disorder and no abnormal coagulation profile before conception. Pregnancy, therefore, can act as the “first stress test” for an unrecognized clotting tendency.

This evidence supports a symptom-driven, not history-driven, standard of care. Bleeding with clots after the first trimester, unexplained pain, sudden swelling, or decreased fetal movement should trigger objective evaluation—including coagulation studies and placental imaging—regardless of prior medical history, when appropriate.

Be Your Own Advocate

How can I speak up for myself and make sure my concerns are taken seriously?

Advocating for yourself during pregnancy means trusting your instincts and not allowing your symptoms to be dismissed when something doesn’t feel right. You know your body better than anyone else—you recognize when something changes. If you develop unexplained pain, swelling, shortness of breath, or bleeding, it warrants attention and objective evaluation, not reassurance alone.

A responsible physician who truly values your health will never minimize your concerns or discourage simple, noninvasive testing. Tests like D-dimer screening, Doppler ultrasound, or echocardiography exist to identify complications before they become emergencies, and when results are normal, they offer reassurance and clarity—not waste.

A compassionate provider understands that worry about potential clotting or pregnancy complications is both rational and protective. Your vigilance is not overreaction; it is part of good prenatal care. You have every right to ask questions, request testing, and insist on being heard—because your intuition and advocacy are as vital to a safe pregnancy as any medical intervention.

Request evaluation even without prior clotting history

If you experience bleeding with clots, unexplained pain, or reduced fetal movement, request coagulation testing and placental imaging. Normal lab results earlier in pregnancy do not exclude a developing hypercoagulable state.

Ask for targeted thrombophilia testing

Testing for factor V Leiden, prothrombin G20210A mutation, protein C and protein S deficiencies, antithrombin deficiency, antiphospholipid syndrome (APS), and homocysteine levels can reveal underlying risk. The New England Journal of Medicine (2023) notes that identification of these markers allows for timely prophylaxis that can prevent up to 70–80% of recurrent pregnancy losses.

Request serial monitoring of blood and placental flow

Periodic measurement of D-dimer, fibrinogen, and platelet count, combined with Doppler ultrasound of uterine and umbilical arteries, helps detect early placental resistance and hypoperfusion. According to the Maternal Thrombosis Surveillance Project (2024), serial Doppler monitoring reduced the risk of stillbirth by 35% among symptomatic patients who were previously undiagnosed.

Discuss preventive therapies with your provider

Low-dose aspirin (81 mg daily): Shown to reduce placental microthrombi and improve uterine artery perfusion by inhibiting platelet activation.
Low-molecular-weight heparin (LMWH): Proven to improve live-birth rates in thrombophilic pregnancies by up to 80% when started early under maternal-fetal medicine supervision (NEJM, 2023).These therapies are safe when monitored and tailored to the individual’s coagulation profile.

Optimize systemic vascular health

Maintaining hydration, light movement, and an anti-inflammatory diet rich in omega-3 fatty acids and antioxidants supports vascular function. Managing insulin resistance, thyroid imbalance, and chronic inflammation further reduces hypercoagulability.

Advocate persistently if symptoms are minimized

If your symptoms are downplayed or dismissed as “normal pregnancy changes,” trust your instincts—you know your body best. Too many women experience delayed or missed diagnoses because legitimate warning signs are attributed to benign causes such as hormonal shifts, ligament pain, or “typical swelling.” In reality, these can be early indicators of serious thrombotic complications such as deep vein thrombosis (DVT), pulmonary embolism (PE), or placental thrombosis.

If you notice sudden leg pain, unilateral swelling, visible veins, shortness of breath, chest pressure, or unexplained bleeding with clots, do not accept reassurance without objective evaluation. Request a D-dimer test, Doppler ultrasound, or chest imaging—each of which can safely identify or rule out clot formation during pregnancy. Persistent swelling or calf pain should never be dismissed as “normal water retention,” and chest heaviness should not be blamed on reflux or anxiety until vascular causes are excluded.

According to Circulation (2023), one in three pregnant women who later develop DVT or PE report having been told their initial symptoms were “nothing to worry about.” This kind of medical gaslighting—where a patient’s concerns are invalidated or minimized—remains a major barrier to maternal safety and timely diagnosis. Document every symptom, photograph any visible clotting or bleeding, and record the timing and duration of episodes. Bring these records to each appointment and, if necessary, request escalation to a maternal-fetal medicine (MFM) specialist or hematologist familiar with high-risk pregnancies.

If something feels wrong, insist on answers, testing, and specialist review. You are the expert on your own body, and early, evidence-based persistence can prevent life-threatening complications for both you and your baby.

Pregnancy is the most prothrombotic state many women will ever experience, and a normal history does not guarantee normal risk. Evidence across major obstetric and hematologic journals confirms that nearly half of all clot-related pregnancy complications occur in women with no prior diagnosis or family history. Any bleeding with clots after mid-pregnancy warrants immediate evaluation, as it may indicate placental microthrombosis rather than benign bleeding. With early recognition, targeted testing, and preventive anticoagulation, outcomes are overwhelmingly positive—transforming what could be a silent placental disorder into a preventable event for both mother and child.

Signs of Deep Vein Thrombosis (DVT)

What early warning signs suggest a clot may have formed in a leg or arm?

Deep vein thrombosis (DVT) occurs when a clot forms in the deep veins of the legs or arms, blocking normal blood flow. Common warning signs include:

Swelling in the affected leg or arm, often noticeable at the ankle or calf
Pain or tenderness that is not linked to an injury and worsens with standing or walking
Warm, red, or discolored skin over the affected area

If you experience any of these symptoms, contact your healthcare provider immediately. Early diagnosis and treatment dramatically reduce the risk of complications.

Signs of Pulmonary Embolism (PE)

When does a blood clot become a life-threatening emergency?

When a clot breaks free and travels to the lungs, it becomes a pulmonary embolism (PE), a life-threatening emergency that requires immediate medical care. Symptoms may include:

Sudden shortness of breath or difficulty breathing
Chest pain that worsens with deep breathing or coughing
Coughing up blood or a persistent dry cough
Rapid or irregular heartbeat

If these symptoms occur, call emergency services right away. Delaying care can be fatal. Research in The American Journal of Obstetrics & Gynecology (2021) found that up to 25% of pregnancy-related PE cases present as sudden cardiac arrest without prior warning, underscoring the need for immediate response to any suspicious symptoms.

Discuss Your Risk Factors with a Healthcare Provider

Why should each patient have an individualized plan for clot prevention?

Every pregnancy is unique. Speak with your clinician about any personal or family history of blood clots, as well as conditions that may increase your risk, such as obesity, inherited thrombophilias, or prolonged bed rest. This conversation is critical for tailoring a prevention plan suited to your individual health profile.

Medical Guidance During and After Pregnancy

How do anticoagulants, such as low-molecular-weight heparin, protect both the mother and the child?

For individuals at higher risk, or those who develop a clot during pregnancy, doctors often prescribe low-molecular-weight heparin (LMWH), an injectable anticoagulant proven safe for both mother and baby. Studies published in Blood Advances (2020) confirm that LMWH effectively prevents recurrent venous thromboembolism (VTE) without increasing the risk of significant bleeding or affecting fetal development.

Administered under the skin, LMWH is used throughout pregnancy and may continue for several weeks postpartum, when clot risk remains highest. Close coordination with your healthcare provider ensures proper dosing, timing, and monitoring throughout treatment.

Staying informed, recognizing early symptoms, and adhering to preventive care can make all the difference, helping both you and your baby stay safe during pregnancy and beyond.

The International Women’s Blood Clot Alliance (IWBCA) provides the information and materials on this site for educational and informational purposes only. The content is not a substitute for professional medical evaluation, diagnosis, or treatment. Always consult your physician or another qualified healthcare provider regarding any questions you may have about a medical condition, diagnosis, or course of treatment. Do not disregard, delay, or alter medical advice based on information obtained from this site. If you believe you are experiencing a medical emergency, call 911 or your local emergency services immediately.