How should anticoagulation be managed, including bridging, in a pregnant patient with a mechanical heart valve, prior venous thrombo‑embolism, antiphospholipid syndrome, or high‑risk atrial fibrillation?

Bridging Anticoagulation in Pregnancy

Pregnant patients requiring anticoagulation must receive continuous therapeutic anticoagulation with frequent monitoring throughout pregnancy, with the specific regimen determined by the indication (mechanical heart valve, VTE, antiphospholipid syndrome, or atrial fibrillation) and gestational age, recognizing that no strategy is optimally safe for both mother and fetus. 1

Mechanical Heart Valves: The Highest Risk Scenario

Critical Foundation

• All pregnant patients with mechanical prosthetic valves require uninterrupted therapeutic anticoagulation throughout pregnancy—this is non-negotiable. 1
• Maternal mortality remains >1% even with optimal management, and more than one-third experience serious maternal or fetal complications. 1
• Women unable to maintain therapeutic anticoagulation with frequent monitoring should be counseled against pregnancy due to prohibitive risk of catastrophic valve failure or stroke. 1

First Trimester Management (Weeks 6-12)

The choice depends critically on warfarin dose requirements:

If Warfarin Dose ≤5 mg/day:

• Continue warfarin throughout all three trimesters (this minimizes maternal risk and is reasonable given lower embryopathy risk at this dose). 1
• Alternative: Switch to dose-adjusted LMWH twice daily (target anti-Xa 0.8-1.2 U/mL at 4-6 hours post-dose) during weeks 6-12, then resume warfarin. 1, 2
• Alternative: Continuous IV unfractionated heparin (aPTT ≥2 times control) during weeks 6-12. 1

If Warfarin Dose >5 mg/day:

• Switch to dose-adjusted LMWH twice daily (target anti-Xa 0.8-1.2 U/mL at 4-6 hours post-dose) during weeks 6-12, then resume warfarin for second and third trimesters. 1, 2
• Alternative: Continuous IV unfractionated heparin (aPTT ≥2 times control) during weeks 6-12. 1

The rationale: Warfarin embryopathy risk is dose-dependent and particularly high >5 mg/day during organogenesis (weeks 6-12). 1

Second and Third Trimesters (Weeks 13-36)

• Resume warfarin with target INR 3.0 (range 2.5-3.5) regardless of first trimester strategy. 1, 2
• Warfarin is safest for the mother during this period, as LMWH carries higher maternal thrombotic risk. 1

Peripartum Management (Week 36 Onward)

This is where "bridging" truly occurs:

• At 36 weeks gestation: Discontinue warfarin and switch to continuous IV UFH (aPTT ≥2 times control) or dose-adjusted LMWH twice daily. 1, 2
• At 36 hours before planned delivery: Switch from LMWH to continuous IV UFH if not already on it. 1
• 4-6 hours before planned delivery: Stop IV heparin. 1
• If labor begins while on warfarin: Perform cesarean section after reversal of anticoagulation. 1
• Postpartum: Resume UFH 4-6 hours after delivery and begin oral warfarin in absence of significant bleeding. 1

Critical Monitoring Requirements for LMWH

• LMWH must be given twice daily, never once daily, for therapeutic anticoagulation. 1, 2
• Anti-Xa levels must be monitored 4-6 hours after morning dose, targeting 0.7-1.2 U/mL. 1, 2
• Never use LMWH in mechanical valve patients without anti-Xa monitoring—valve thrombosis deaths have occurred with inadequate monitoring. 1, 2
• Dose adjustments are needed in approximately 45% of cases as pregnancy progresses due to changing volume of distribution. 2

Venous Thromboembolism (VTE)

Acute VTE During Pregnancy

• Use therapeutic-dose LMWH twice daily throughout pregnancy (this is strongly preferred over UFH). 3, 4
• Continue anticoagulation for at least 6 weeks postpartum, with total minimum duration of 6 months. 3
• Target anti-Xa level 0.8-1.2 U/mL at 4-6 hours post-dose. 2

History of Prior VTE

Single Prior VTE with Transient Risk Factor (No Longer Present, No Thrombophilia):

• Clinical surveillance antepartum plus anticoagulant prophylaxis postpartum. 3

Single Prior VTE with Higher-Risk Thrombophilia:

• Antepartum prophylactic or intermediate-dose LMWH plus postpartum anticoagulation. 3
• Higher-risk thrombophilias include antithrombin deficiency, homozygous factor V Leiden, or antiphospholipid syndrome. 3

Multiple Prior VTE Episodes:

• Antepartum prophylactic, intermediate-dose, or adjusted-dose LMWH followed by postpartum anticoagulation. 3

Currently on Long-Term Anticoagulation for VTE:

• Switch to adjusted-dose LMWH or UFH throughout pregnancy, then resume long-term anticoagulation postpartum. 3

Antiphospholipid Syndrome

With Recurrent Pregnancy Loss (No History of Thrombosis)

• Prophylactic or intermediate-dose UFH or prophylactic LMWH combined with aspirin throughout pregnancy. 3
• Screen all women with recurrent early pregnancy loss or unexplained late pregnancy loss for antiphospholipid antibodies. 3

With History of Thrombosis

• Treat as VTE with therapeutic-dose LMWH throughout pregnancy. 3

Atrial Fibrillation

High-Risk AF Requiring Anticoagulation

• Discontinue warfarin between weeks 6-12 and use LMWH twice daily with dose adjustment targeting anti-Xa level 0.8-1.2 U/mL. 2
• Resume warfarin for second and third trimesters if patient values favor maternal safety. 2
• Avoid all NOACs (dabigatran, rivaroxaban, apixaban) completely during pregnancy—these are absolutely contraindicated. 2

Common Pitfalls and How to Avoid Them

Dosing Errors

• Never use once-daily LMWH for therapeutic anticoagulation in pregnancy—twice-daily dosing is mandatory for adequate levels. 1, 2
• Do not assume non-pregnant LMWH doses are adequate—pregnancy increases volume of distribution requiring dose escalation in 45% of patients. 2

Monitoring Failures

• LMWH without anti-Xa monitoring in mechanical valve patients has resulted in maternal deaths from valve thrombosis. 1
• Anti-Xa levels must be checked 4-6 hours post-dose, not at trough. 1, 2

Timing Errors

• Warfarin embryopathy risk is highest weeks 6-12, but CNS abnormalities can occur with any trimester exposure. 2
• Switching from warfarin to heparin too close to delivery risks maternal valve thrombosis; switching too early increases time on less effective anticoagulation. 1

Shared Decision-Making Failures

• Physicians must not assume a woman's values or supplant their own preferences—some women prioritize maternal safety (favoring warfarin), others prioritize fetal safety (favoring heparin). 1
• The discussion must explicitly state that no anticoagulation strategy is optimally safe for both mother and fetus. 1

Drug Selection Errors

• Fondaparinux should only be used for severe heparin allergy with HIT—it is not a routine alternative. 2
• UFH carries higher risks of thrombocytopenia and osteoporosis compared to LMWH. 1, 5, 4