Anticoagulation in pregnancy: Q&A on low molecular weight heparin

• Leave the patient on LMWH until the onset of labor. Depending on the timing and dose of her last injection, she may or may not be a candidate for regional anesthesia in labor. With prophylactic dosing, the risk of significant hemorrhage is low.
• Time delivery so that the patient withholds her PMinjection and presents for delivery the following morning, 24 hours after her last dose. This option is attractive when cesarean delivery is planned. The advantage of this approach is that it keeps the window of thrombosis as narrow as possible. The disadvantage is that the patient may enter labor or require delivery before the chosen delivery date, in which case she may be at risk for hemorrhage.
• Convert the patient to UH at approximately 36 to 37 weeks’ gestation. The advantage? The shorter half-life of UH increases the possibility for regional anesthesia and decreases the risk for hemorrhage. The disadvantage: The risk for thrombosis as well as complications (thrombocytopenia, osteoporosis, thrombosis) may be greater with UH for the reasons described earlier.

Restarting after delivery. Strategies for continued thromboprophylaxis in the postpartum period include continuing LMWH for an additional 6 to 8 weeks or converting to warfarin. Warfarin can be taken orally but requires frequent monitoring and adjustment of dosing, which is unnecessary with LMWH. Many patients choose to continue LMWH because they are familiar with the routine of daily injections. There is little data to support or refute either approach.

There also is little data to guide the decision of when to reinitiate prophylactic or therapeutic doses of LMWH after vaginal or cesarean delivery. Translating from other surgical subspecialties, most obstetricians are comfortable resuming both prophylactic and therapeutic doses at 6 hours after vaginal delivery and 8 hours after cesarean section.

Question 9How do I monitor the effectiveness of LMWH?

The frequent monitoring necessary with UH is not required with LMWH, since the increased bioavailability of LMWH leads to reliable tissue levels. The aPTT level does not correlate well with the anticoagulation effect of LMWH.

Antifactor Xa levels—sometimes referred to as the LMWH assay—of 0.5 to 1.2 IU/mL are considered adequate for therapeutic anticoagulation in the nonpregnant population. Peak anti-Xa activity is achieved approximately 4 hours after subcutaneous injection. AntiXa levels should therefore be drawn 4 hours after the last dose.¹⁷ Prophylactic subcutaneous dosing of enoxaparin 40 mg daily generally does not require monitoring of anti-Xa levels.

In nonpregnant women, routine monitoring is seldom necessary. Gravidas, however, are constantly changing in terms of weight, plasma volume, renal clearance, and amount of heparin-binding proteins. Because of this, periodic monitoring (every 4 weeks) is reasonable until an evidence-based recommendation can be made.

Question 10When can I provide neuraxial anesthesia?

Most anesthesiologists are reluctant to perform neuraxial anesthesia within 12 hours of the last prophylactic dose or 24 hours of the last therapeutic dose of LMWH because of the risk of epidural and spinal hematoma.

The American Society of Regional Anesthesia recommends that neuraxial anesthesia be withheld for 24 hours after the last therapeutic dose and 12 hours after a prophylactic dose. That organization did not recommend checking anti-Xa levels, since they do not adequately predict the risk of bleeding.⁹ However, as experience with LMWH broadens in nonobstetric surgical cases, such as orthopedic and cardiac procedures, it is likely that greater familiarity with the medication will lead to better evidence and broader acceptance.

Dr. Emery reports no financial relationships relevant to this article.