Is heparin (subcutaneous) safe during pregnancy?

Subcutaneous Heparin Safety in Pregnancy

Yes, subcutaneous heparin (both unfractionated heparin and low-molecular-weight heparin) is safe for use during pregnancy because it does not cross the placental barrier and poses no teratogenic risk to the fetus. 1, 2

Fetal Safety Profile

Heparin is the anticoagulant of choice during pregnancy due to its inability to cross the placenta, eliminating any direct fetal exposure or risk of teratogenicity. 3, 2, 4

• Neither unfractionated heparin (UFH) nor low-molecular-weight heparin (LMWH) cross the placental barrier, and neither is found in breast milk in significant amounts 3
• No risk of fetal bleeding complications exists with heparin therapy, unlike warfarin which can cause fetal cerebral hemorrhage 2
• Heparin has no teratogenic potential, whereas warfarin causes embryopathy in 4-10% of exposed pregnancies 2, 4
• In pregnant women without comorbid conditions treated with LMWH, adverse fetal outcomes occurred in only 3.1% of pregnancies, comparable to the normal population 5

Preferred Formulation

Low-molecular-weight heparin is strongly preferred over unfractionated heparin for both prevention and treatment of venous thromboembolism in pregnancy (Grade 1B recommendation). 1

• LMWH offers significant advantages including no monitoring requirement and reduced risk of osteoporosis and heparin-induced thrombocytopenia compared to UFH 3
• LMWH administered once daily has been proven safe and effective for thromboprophylaxis during pregnancy and postpartum 6
• The typical LMWH dose is either 200 IU/kg once daily or 100 IU/kg twice daily 3

Maternal Considerations and Monitoring

While safe for the fetus, heparin therapy requires attention to maternal risks:

• Long-term UFH therapy carries occasional risk of maternal hemorrhage and rarely symptomatic osteoporosis 7
• For therapeutic LMWH, measure anti-Xa levels 4-6 hours after morning dose, targeting 0.7-1.2 units/mL 1, 2
• For UFH given subcutaneously twice daily, target aPTT should be 1.5-2.5 times control, measured 6 hours after injection 3
• Monitor platelet counts throughout therapy regardless of route of administration to detect heparin-induced thrombocytopenia 8

Critical Peripartum Management

Discontinue LMWH at least 24 hours before planned induction of labor, cesarean section, or neuraxial anesthesia (Grade 1B) to reduce hemorrhagic complications. 1

• Subcutaneous heparin injections should be discontinued at the onset of regular uterine contractions 3
• Women who delivered within 28 hours of their last heparin injection had elevated aPTTs at delivery, increasing bleeding risk 9
• The use of epidural analgesia in patients who received heparin within the previous 12-24 hours is debated due to epidural hematoma risk, though this risk is probably very low 3

Important Contraindication

Do NOT use heparin formulations preserved with benzyl alcohol in pregnant women, as benzyl alcohol is associated with serious adverse events. 8

• Use only preservative-free heparin sodium injection in pregnancy 8
• Benzyl alcohol has been associated with the "gasping syndrome" characterized by CNS depression, metabolic acidosis, and potentially death 8

Postpartum Anticoagulation

• All pregnant women with a history of VTE require postpartum prophylaxis for 6 weeks with either LMWH at prophylactic or intermediate doses, or vitamin K antagonists with INR target of 2.0-3.0 (Grade 2B) 1
• Warfarin can be safely used postpartum by breast-feeding mothers 7
• For women receiving anticoagulation for acute VTE treatment, continue anticoagulants for at least 6 weeks postpartum for a minimum total duration of 3 months (Grade 2C) 1

Common Pitfall to Avoid

The major pitfall is using heparin too close to delivery time. A conservative approach requires discontinuing subcutaneous heparin 24 hours prior to elective induction to avoid a prolonged aPTT at delivery and associated bleeding complications 9. Six of 11 women in one study delivered with prolonged aPTTs when this timing was not strictly observed, with one experiencing major bleeding during emergency cesarean section 9.