Can Low Molecular Weight Heparin (LMWH) be used after 36 weeks of gestation in patients with a prosthetic valve replacement?

LMWH Use After 36 Weeks in Prosthetic Valve Patients

No, LMWH should NOT be used after 36 weeks of gestation in pregnant patients with prosthetic valves—warfarin must be discontinued and continuous intravenous unfractionated heparin (UFH) initiated at 36 weeks to prepare for delivery. 1, 2

Critical Transition at 36 Weeks

At exactly 36 weeks gestation, the anticoagulation strategy must change regardless of what was used earlier in pregnancy:

• Warfarin must be stopped immediately to prevent catastrophic fetal intracranial hemorrhage during labor and delivery 1, 2
• LMWH must also be discontinued at 36 weeks due to inability to rapidly reverse anticoagulation and documented maternal deaths from valve thrombosis when used inappropriately 2
• Continuous intravenous UFH is the only acceptable anticoagulant from 36 weeks until delivery 1, 2

Why UFH is Mandatory After 36 Weeks

The ACC/AHA guidelines explicitly state that warfarin should be discontinued and continuous intravenous UFH given starting 2-3 weeks before planned delivery 1:

• UFH does not cross the placenta, eliminating fetal anticoagulation and hemorrhage risk during delivery 2
• UFH has a short half-life of 60-90 minutes, allowing rapid reversal with protamine if emergency delivery or bleeding occurs 2
• LMWH cannot be rapidly reversed and has been associated with maternal valve thrombosis deaths when used at this stage 2

The "Up to 36 Weeks" Language Explained

The guidelines state that LMWH, UFH, or warfarin can be used "up to 36 weeks"—this means UNTIL 36 weeks, not AFTER 36 weeks 1:

• Before 36 weeks: dose-adjusted LMWH twice daily (with anti-Xa monitoring 0.7-1.2 U/mL), continuous IV UFH, or warfarin are all acceptable options 1
• At 36 weeks: immediate transition to continuous IV UFH is mandatory 1, 2

Specific Protocol at 36 Weeks

Target aPTT at least 2 times control (typically 60-80 seconds) with continuous IV UFH 1, 2:

• Check aPTT every 4-6 hours initially until stable therapeutic range achieved 2
• Plan delivery between 37-39 weeks 2
• Stop UFH infusion 4-6 hours before planned delivery 1, 2
• Resume UFH 4-6 hours after delivery if no significant bleeding 1, 2
• Restart warfarin on postpartum day 1 if hemostasis adequate 2

Evidence Against LMWH After 36 Weeks

Multiple treatment failures and maternal deaths have been documented with LMWH in prosthetic valve patients:

• In one study, 2 of 8 pregnant women on enoxaparin 1 mg/kg twice daily developed prosthetic valve thromboses leading to maternal and fetal death 1
• Valve thrombosis occurred in 8.64% of pregnancies using LMWH, with most complications in patients receiving fixed doses without anti-Xa monitoring 3
• Case reports document embolic myocardial infarction and valve thrombosis even with therapeutic LMWH 4, 5, 6

Common Pitfalls to Avoid

Never continue warfarin beyond 36 weeks due to high risk of fetal intracranial hemorrhage during labor 1, 2:

• Warfarin crosses the placenta and anticoagulates the fetus, creating catastrophic bleeding risk during delivery 2

Never use LMWH at 36+ weeks as a bridge to delivery:

• LMWH cannot be rapidly reversed if emergency delivery needed 2
• The longer half-life of LMWH (compared to UFH) creates unacceptable bleeding risk during labor 2

Never use subcutaneous UFH instead of continuous IV UFH at this stage:

• Subcutaneous UFH causes persistent anticoagulant effect at delivery, complicating labor management 1
• Only continuous IV UFH allows precise control and rapid discontinuation 1, 2

Additional Considerations

Consider adding low-dose aspirin 75-100 mg daily if not already prescribed, as this is reasonable in the second and third trimesters for mechanical valve patients 1:

• Aspirin provides additional antiplatelet protection without significantly increasing bleeding risk 1