How long should low molecular weight heparin (LMWH) be administered to pregnant women with deep vein thrombosis (DVT)?

Duration of Heparin Therapy for DVT in Pregnant Women

Pregnant women diagnosed with acute DVT should receive therapeutic anticoagulation throughout the remainder of pregnancy and continue for at least 6 weeks postpartum, with a minimum total treatment duration of 3 months. 1

Treatment Algorithm

Antepartum Period

• Initiate adjusted-dose LMWH immediately upon DVT diagnosis and continue throughout the entire pregnancy 1
• Use weight-adjusted therapeutic dosing: enoxaparin 1 mg/kg every 12 hours or dalteparin 100 units/kg every 12 hours 1
• LMWH is strongly preferred over unfractionated heparin for both efficacy and safety 1

Peripartum Management

• Discontinue LMWH at least 24 hours before planned delivery (induction of labor, cesarean section, or neuraxial anesthesia) 1
• This timing is critical to minimize bleeding risk while maintaining thromboprophylaxis 1

Postpartum Period

• Resume anticoagulation 8-12 hours after delivery and continue for at least 6 weeks postpartum 2
• The total duration of anticoagulation must be at least 3 months from the time of DVT diagnosis 1
• Postpartum options include:
  • Continue LMWH at prophylactic or intermediate doses for 6 weeks 1
  • Transition to warfarin (INR target 2.0-3.0) with initial LMWH overlap until therapeutic INR is achieved 1
  • Both LMWH and warfarin are safe during breastfeeding 1

Key Clinical Considerations

Calculating Total Duration

The 3-month minimum duration starts from DVT diagnosis, not from delivery 1. For example:

• DVT diagnosed at 30 weeks gestation requires treatment through delivery plus 6 weeks postpartum (approximately 16 weeks total)
• DVT diagnosed at 8 weeks gestation requires treatment through delivery plus only the standard 6 weeks postpartum (approximately 38 weeks total)

Evidence Quality

The recommendation for 3-month minimum duration with 6-week postpartum extension is based on moderate-quality evidence extrapolated from non-pregnant populations, as high-quality randomized trials in pregnancy are limited 1. The American College of Chest Physicians guidelines acknowledge this represents Grade 2C evidence (weak recommendation, low-quality evidence) 1.

Common Pitfalls to Avoid

• Do not stop anticoagulation at delivery - the postpartum period carries the highest VTE risk, and premature discontinuation significantly increases recurrence risk 1
• Do not use warfarin during pregnancy - it causes embryopathy and fetal bleeding complications; reserve for postpartum period only 1
• Do not continue LMWH up to the time of delivery - this increases hemorrhagic complications and prevents safe neuraxial anesthesia 1

Dosing Considerations

Recent evidence from the Highlow trial suggests that fixed low-dose LMWH may be as effective as weight-adjusted intermediate-dose for preventing recurrent VTE in women with prior VTE history, though this study focused on prophylaxis rather than acute treatment 3. For acute DVT treatment, therapeutic weight-adjusted dosing remains the standard 1.