Understanding women's blood clot and bleeding disorder risks during pregnancy
Pregnancy is one of the most hemostasis-intensive experiences of a woman's life. The body must navigate two competing demands simultaneously: protecting against blood clot risk in women as the cardiovascular system adapts to pregnancy, and preparing for the blood loss that comes with delivery.
Understanding how women's bleeding and clotting interact across pregnancy, birth and the postpartum period is essential for both patients and healthcare professionals. For women with pre-existing conditions affecting women's hemostasis, that balance requires especially careful, expert management.
How pregnancy changes your blood's clotting system to protect you
Pregnancy naturally changes the way your blood clots, and that's by design. Starting in the first trimester, your body increases certain clotting proteins, including fibrinogen, factor VIII and von Willebrand factor, while reducing others that normally prevent clots. Your blood also becomes slower to break down clots on its own. These changes build gradually and reach their peak near the end of pregnancy.¹
This shift in your blood's clotting system is not a problem. It's protection. Your body is preparing to control bleeding when the placenta separates during delivery, reducing the risk of serious blood loss. For most women, these changes return to normal within four to six weeks after giving birth.²
But the same changes that guard against heavy bleeding can also raise your risk of developing a blood clot during pregnancy and in the weeks after delivery. This balance between bleeding risk and clot risk is what makes blood health so important to monitor throughout the perinatal period. Women with a bleeding or clotting disorder, or a personal history of venous thromboembolism (VTE), may need a personalized care plan starting early in pregnancy.
Explore curated resources
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Haemostatic changes in pregnancy - (Thrombosis Research) A foundational peer-reviewed review of the coagulation and fibrinolytic changes that occur across normal pregnancy and their clinical implications.
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Hemostasis during normal pregnancy and puerperium - (Seminars in Thrombosis and Hemostasis) A detailed clinical overview of hemostatic changes from conception through the postpartum period, including reference ranges and their normalization after delivery.
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Managing the coagulopathy of postpartum hemorrhage: an evolving role for viscoelastic hemostatic assays (Journal of Thrombosis and Haemostasis) - An ISTH journal article examining the use of point-of-care viscoelastic testing to guide hemostatic management during postpartum hemorrhage.
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Foundation for Women and Girls with Blood Disorders (FWGBD) Patient Resources - Patient resources. Downloadable patient guides, period toolkits and provider-patient resources focused on bleeding disorders in women and girls across the lifespan.
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ASH Maternal Health Care Resources - A collection of resources for patients and clinicians on the intersection of hematologic conditions and maternal health, including hemorrhage and VTE in pregnancy.
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ASH patient page: Bleeding disorders - A plain-language overview of common bleeding disorders, including von Willebrand disease and hemophilia, from the American Society of Hematology.
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Thrombotic risk in pregnancy:
Why it rises and when it matters
Pregnant women face a five to tenfold increase in VTE risk compared with non-pregnant women of the same age, driven by reduced blood flow, the procoagulant shift in women's hemostasis and vascular injury at delivery.³ Thrombosis in women during pregnancy follows a distinct pattern: deep vein thrombosis is more common before delivery, while pulmonary embolism is more frequent after. Around 80% of postpartum thromboembolic events occur within the first three weeks after birth, though blood clot risk in women remains elevated for up to 12 weeks.⁴
Compounding risk factors include prior VTE, inherited thrombophilia, cesarean delivery, immobility, obesity and advanced maternal age. Diagnosis is further complicated by the fact that common VTE symptoms, including leg swelling and shortness of breath, overlap with normal pregnancy changes. Pulmonary embolism accounts for approximately 9% of pregnancy-related deaths in the United States, making early risk identification essential.⁵
Explore curated resources
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VTE risk assessment in pregnancy - (Research and Practice in Thrombosis and Haemostasis) An ISTH Congress State of the Art review covering VTE epidemiology, risk stratification and prevention strategies in pregnancy.
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Venous thromboembolism risk score and pregnancy (Frontiers in Cardiovascular Medicine) - A clinical review of VTE risk scoring tools and thromboprophylaxis recommendations across the perinatal period.
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Venous thromboembolism risk score and pregnancy JACC Focus Seminar (Journal of the American College of Cardiology). A comprehensive clinical overview of VTE risk, diagnosis and treatment in pregnancy and the postpartum period.
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World Thrombosis Day - The global campaign of the ISTH dedicated to raising awareness and understanding of blood clots. Access educational materials, patient-friendly resources, stories and tools to help you understand and reduce your risk of life-threatening blood clots.
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VLN Network: Pregnancy and Blood Clots: What you need to know. - Plain-language information on blood clot risks, symptoms and treatment, including specific guidance for women and pregnancy.
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Inherited bleeding and clotting disorders in pregnancy
Women with inherited conditions affecting women's bleeding and clotting, including von Willebrand disease, hemophilia carriership, rare clotting factor deficiencies, inherited platelet disorders and hereditary thrombophilias, face heightened complexity throughout pregnancy and delivery.
VWD is the most commonly diagnosed inherited bleeding disorder in women. While VWF levels and factor VIII typically rise during pregnancy, they may not reach sufficient levels in type 2 or type 3 VWD, and fall rapidly after delivery in all types, raising the risk of postpartum maternal hemorrhage.⁹ Hemophilia carriers face similar postpartum risks. For women with hereditary thrombophilia, including factor V Leiden or prothrombin gene mutations, pregnancy amplifies blood clot risk in women, often necessitating thromboprophylaxis.⁴
Management should begin before conception and involve a multidisciplinary team including a hematologist, obstetrician and anesthesiologist. Delivery should be planned at a center with hemostasis expertise and access to factor concentrates, desmopressin and tranexamic acid. Every woman with an inherited bleeding or clotting disorder affecting women's hemostasis should have an individualized written care plan in place well before her due date.¹⁰
Explore curated resources
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ASH ISTH NHF WFH 2021: guidelines on the management of von Willebrand disease (Blood Advances) - Joint society evidence-based guidelines on VWD diagnosis and management, including pregnancy-specific recommendations.
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How I manage pregnancy in carriers of hemophilia and patients with von Willebrand disease (Blood) - Practical clinical guidance on multidisciplinary management of inherited bleeding disorders across pregnancy and delivery.
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ISTH SSC reference tools, including the Bleeding Assessment Tool. Standardized clinical tools developed by the ISTH Scientific and Standardization Committee to support diagnosis and management in hemostasis.
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World Thrombosis Day - Annual global awareness campaign with patient resources on thrombosis risk, including for women with inherited clotting conditions.
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Foundation for Women and Girls with Blood Disorders: Patient resources - Downloadable guides, period toolkits and provider-patient resources for women with bleeding disorders, including pregnancy-related content.
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ASH patient page: Bleeding disorders - Patient-friendly overview of inherited bleeding disorders including VWD and hemophilia, with information on symptoms, diagnosis and treatment.
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National Bleeding Disorders Foundation - Women with bleeding disorders Education and advocacy resources for women living with inherited bleeding disorders, including guidance on pregnancy and reproductive health.
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Postpartum hemorrhage: preventing bleeding while managing clot risk
Postpartum hemorrhage, or PPH, is the leading cause of maternal death worldwide, accounting for approximately one quarter of all maternal deaths globally.⁷ It is defined as blood loss of 500 milliliters or more after vaginal delivery, or 1,000 milliliters or more following cesarean delivery. The primary control mechanism is uterine contraction; when that fails, bleeding escalates rapidly and the clotting system can break down.
Managing PPH presents one of the most difficult tensions in perinatal women's hemostasis: anticoagulant therapies used to reduce blood clot risk in women can increase hemorrhage risk at delivery, while undertreating thrombotic risk carries its own serious consequences. Fibrinogen, which rises during pregnancy, falls rapidly and critically during hemorrhage before other clotting factors are affected, making it a key early marker of deterioration. ISTH guidance supports real-time coagulation monitoring, including point-of-care viscoelastic hemostatic assays, to guide hemostatic management during PPH.¹
Explore curated resources
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GUIDELINES: Management of coagulopathy associated with postpartum hemorrhage: guidance from the SSC of the ISTH. Journal of Thrombosis and Haemostasis, 2016) - ISTH SSC guidance on coagulation monitoring and blood product replacement strategies during PPH
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Managing the coagulopathy of postpartum hemorrhage: an evolving role for viscoelastic hemostatic assays (Journal of Thrombosis and Haemostasis. 2023) - Clinical review of VHA use in PPH, including fibrinogen monitoring, goal-directed resuscitation and institutional implementation.
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Postpartum hemorrhage (Maternal Fetal Medicine, 2025) - A comprehensive 2025 review of PPH epidemiology, pathophysiology, hemostatic management and global disparities in outcomes.
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FIGO resources on PPH - Resources on postpartum hemorrhage from the International Federation of Gynaecology and Obstetrics
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Foundation for Women and Girls with Blood Disorders (FWGBD) Patient Resources - Patient resources. Downloadable patient guides, period toolkits and provider-patient resources focused on bleeding disorders in women and girls across the lifespan.
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ASH Maternal Health Care Resources - A collection of resources for patients and clinicians on the intersection of hematologic conditions and maternal health, including hemorrhage and VTE in pregnancy.
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ASH patient page: Bleeding disorders - A plain-language overview of common bleeding disorders, including von Willebrand disease and hemophilia, from the American Society of Hematology.
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Pregnancy-related VTE: recognizing, preventing and treating blood clots in girls and women
Pregnancy-related VTE affects an estimated 0.8 to 2.0 per 1,000 deliveries worldwide and remains a leading preventable cause of maternal mortality.³ Deep vein thrombosis in pregnancy most commonly presents in the proximal left iliofemoral veins, reflecting uterine compression of the left iliac vein, a pattern distinct from VTE in non-pregnant individuals. Standard diagnostic algorithms and D-dimer thresholds developed for non-pregnant patients are not directly applicable in pregnancy, requiring clinical awareness and pregnancy-specific approaches to diagnosis.⁴
Low-molecular-weight heparin is the recommended treatment for pregnancy-associated VTE and for thromboprophylaxis in high-risk women. Direct oral anticoagulants are contraindicated in pregnancy due to fetal risk. ISTH and the American Society of Hematology convened a joint expert panel in late 2025 to develop updated, evidence-based guidelines on VTE management in pregnancy.⁸ Blood clot risk in women should be formally assessed at the start of pregnancy, at delivery and whenever risk factors change.
Explore curated resources
GUIDELINES: ISTH and ASH joint VTE in pregnancy. Announcement of the new joint ISTH/ASH expert panel convened to develop updated pregnancy VTE guidelines
World Thrombosis Day - The global campaign of the ISTH dedicated to raising awareness and understanding of blood clots. Access educational materials, patient-friendly resources, stories and tools to help you understand and reduce your risk of life-threatening blood clots.
CDC Stop the Clot, Spread the Word: Blood clots and pregnancy -Patient videos and toolkits on recognizing and preventing blood clots during pregnancy, developed in collaboration with the CDC.
VLN Network: Pregnancy and Blood Clots: What you need to know. - Plain-language information on blood clot risks, symptoms and treatment, including specific guidance for women and pregnancy.
Page references:
¹ Brenner B. Haemostatic changes in pregnancy. Thrombosis Research. 2004; 114:409-414. https://pubmed.ncbi.nlm.nih.gov/12763484/
² Hellgren M. Hemostasis during normal pregnancy and puerperium. Seminars in Thrombosis and Hemostasis. 2003; 29:125-130. https://pubmed.ncbi.nlm.nih.gov/12709915/
³ Lim MY, Trujillo TC. Managing the coagulopathy of postpartum hemorrhage: an evolving role for viscoelastic hemostatic assays. Journal of Thrombosis and Haemostasis. 2023. https://www.jthjournal.org/article/S1538-7836(23)00262-3/fulltext
⁴ Philipson J, et al. Incidence and time trends of pregnancy-related first-time venous thromboembolism: a 33-year Swedish Birth Registry study. Journal of Thrombosis and Haemostasis. 2025; 23:2473-2482.
⁵ Ewins K, et al. VTE risk assessment in pregnancy. Research and Practice in Thrombosis and Haemostasis. 2020. https://pmc.ncbi.nlm.nih.gov/articles/PMC7040539/
⁶ Frontiers in Cardiovascular Medicine. Venous thromboembolism risk score and pregnancy. 2022. https://www.frontiersin.org/journals/cardiovascular-medicine/articles/10.3389/fcvm.2022.863612/full
⁷ Nichols KM, Henkin S, Creager MA. Venous thromboembolism associated with pregnancy: JACC Focus Seminar. Journal of the American College of Cardiology. 2020; 76:2128-2141.
⁸ ISTH DIC Scoring Algorithm: Modifications for use in pregnancy. Practical Haemostasis. https://practical-haemostasis.com/Clinical%20Prediction%20Scores/Formulae%20code%20and%20formulae/Formulae/DIC/isth_dic_pregnancy_score.html
⁹ Wang MJ, Oyelese Y. Postpartum hemorrhage. Maternal Fetal Medicine. 2025; 7(1):38-48. https://journals.lww.com/mfm/fulltext/2025/01000/postpartum_hemorrhage.7.aspx
¹⁰ Collins P, Abdul-Kadir R, Thachil J. Management of coagulopathy associated with postpartum hemorrhage: guidance from the SSC of the ISTH. Journal of Thrombosis and Haemostasis. 2016; 14:205-210.
¹¹ ISTH and ASH. Invitation for nominations to serve on management of VTE in the context of pregnancy panel. ISTH newsroom, December 2025. https://www.isth.org/news
¹² Connell NT, et al. ASH ISTH NHF WFH 2021 guidelines on the management of von Willebrand disease. Blood Advances. 2021; 5(1):301-325.
¹³ Leebeek FWG, Duvekot J, Kruip MJHA. How I manage pregnancy in carriers of hemophilia and patients with von Willebrand disease. Blood. 2020; 136(19):2143-2150. https://ashpublications.org/blood/article/136/19/2143/463259