Thromboprophylaxis in pregnancy — risk assessment, tiering and LMWH dosing

It is the booking visit. A 34-year-old woman with a body mass index of 34 had a deep vein thrombosis on the oral contraceptive pill three years ago. She is now eight weeks pregnant and asks whether she needs a blood-thinning injection. The answer is yes — and the dose, the duration, and the postpartum plan are the fellowship-level detail the examiner is listening for. This topic is the prophylaxis pathway: who, with what, at what dose, and for how long. (For the diagnosis and treatment of established VTE in pregnancy, see the dedicated Venous thromboembolism in pregnancy topic.)^[4]

Overview and definition

Thromboprophylaxis in pregnancy is the use of pharmacological and/or mechanical measures to prevent venous thromboembolism (VTE) — deep vein thrombosis and pulmonary embolism. Pregnancy is a prothrombotic state, and VTE complicates around 1 to 2 of every 1000 deliveries, making it a leading cause of direct maternal mortality in high-income countries.^[1][7]

The framework is risk assessment and tiering. Every woman is risk-assessed in early pregnancy and reassessed postpartum, because the risk profile changes — particularly at delivery. The output is a tier (high, intermediate, or low) that maps to a prophylaxis plan: pharmacological LMWH antepartum and/or postpartum, mechanical measures, or expectant management.^[1][4]

Low-molecular-weight heparin (LMWH) is the agent of choice — it does not cross the placenta, is safe in breastfeeding, and has a strong evidence base (ASH 2018 gives a strong recommendation for LMWH over unfractionated heparin).^[1][8]

References11ShowHide

1. [1]Bates SM, Rajasekhar A, Middeldorp S, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: venous thromboembolism in the context of pregnancy Blood Adv, 2018.PMID 30482767
2. [2]Bistervels IM, Buchmüller A, Wiegers HMG, et al. Intermediate-dose versus low-dose low-molecular-weight heparin in pregnant and post-partum women with a history of venous thromboembolism (Highlow study): an open-label, multicentre, randomised, controlled trial Lancet, 2022.PMID 36354038
3. [3]Bruno AM, Allshouse AA, Campbell HM, et al. Weight-Based Compared With Fixed-Dose Enoxaparin Prophylaxis After Cesarean Delivery: A Randomized Controlled Trial Obstet Gynecol, 2022.PMID 36075079
4. [4]American College of Obstetricians and Gynecologists' Committee on Practice Bulletins-Obstetrics ACOG Practice Bulletin No. 196: Thromboembolism in Pregnancy Obstet Gynecol, 2018.PMID 29939938
5. [5]Bates SM, Greer IA, Middeldorp S, et al. VTE, thrombophilia, antithrombotic therapy, and pregnancy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines Chest, 2012.PMID 22315276
6. [6]Ban L, Abdul Sultan A, West J, et al. External validation of a model to predict women most at risk of postpartum venous thromboembolism: Maternity clot risk Thromb Res, 2021.PMID 34120750
7. [7]Andrew L, Ní Áinle F, Blondon M, et al. Preventing postpartum venous thromboembolism: A call to action to reduce undue maternal morbidity and mortality Thromb Res, 2020.PMID 32738644
8. [8]Greer IA, Nelson-Piercy C Low-molecular-weight heparins for thromboprophylaxis and treatment of venous thromboembolism in pregnancy: a systematic review of safety and efficacy Blood, 2005.PMID 15811953
9. [9]Jacobson B, Rambiritch V, Paek D, et al. Safety and Efficacy of Enoxaparin in Pregnancy: A Systematic Review and Meta-Analysis Adv Ther, 2020.PMID 31673991
10. [10]Federspiel JJ, Wein LE, Addae-Konadu KL, et al. Venous thromboembolism incidence among patients recommended for pharmacologic thromboembolism prophylaxis after cesarean delivery in selected guidelines J Thromb Haemost, 2021.PMID 33345404
11. [11]Zheng J, Chen Q, Fu J, et al. Critical appraisal of international guidelines for the prevention and treatment of pregnancy-associated venous thromboembolism: a systematic review BMC Cardiovasc Disord, 2019.PMID 31419948