Anticoagulants and pregnancy: When are they safe?

Author and Disclosure Information




Acute deep venous thrombosis and pulmonary embolism

If acute deep venous thrombosis or pulmonary embolism is confirmed or strongly suspected in a pregnant woman, therapeutic anticoagulation should be started promptly ( Table 4 ). In most cases, the woman should probably be hospitalized, given the complex maternal and fetal concerns that include adequate maternal dosing and the potential for fetal harm in the setting of significant hypoxia.

Anticoagulant therapy should begin as full doses of either LMWH or intravenous unfractionated heparin. We prefer starting with LMWH, as it can be started rapidly with less need for nursing care (eg, no need to start and maintain an intravenous line and monitor the aPTT) and has excellent safety. If LMWH is selected, initial dosing should be based on the current weight ( Table 2 ). Subsequent monitoring of the peak anti-factor-Xa activity levels (ie, 4 hours after the dose) is recommended, with the first level drawn in the first few days of treatment, and repeat levels every 1 to 3 months for the rest of treatment. As mentioned earlier, weight-based dosing has not been systematically evaluated in pregnancy.

If unfractionated heparin is the initial agent, it should be given as a bolus followed by a continuous infusion, ideally utilizing a weight-based nomogram to estimate required doses, with adjustment of the infusion rate to maintain the aPTT at 1.5 to 2.5 times the baseline value (obtained during pregnancy). After several days, the heparin may be switched to LMWH in therapeutic doses ( Table 2 ).

Alternatively, in women approaching term or who cannot afford LMWH, anticoagulation may be continued as adjusted-dose subcutaneous unfractionated heparin, ie, two or three large daily doses of subcutaneous heparin to provide therapeutic levels of anticoagulation. The starting dose can be calculated as the total units of heparin required to maintain full anticoagulation intravenously over 24 hours, given as two or three divided doses ( Table 2 ). The aPTT at the mid-dosing interval (eg, 6 hours after the subcutaneous dose during every-12-hour dosing) should be monitored and the dose adjusted to maintain the aPTT at 1.5 to 2.5 times the baseline value.

A therapeutic level of anticoagulation should be maintained for at least 3 months after an acute thrombotic event during pregnancy, though many physicians prefer to continue full anticoagulation for a total of 6 months. Beyond this interval, if the woman is still pregnant, the anticoagulation may be reduced in intensity, perhaps even to a prophylactic level for the duration of the pregnancy (see discussion below on prior venous thromboembolic events) ( Table 2 ). Peripartum and postpartum anticoagulation are discussed further below.


While all pregnant women are at higher risk of venous thrombosis, the overall incidence of thromboembolism is only about one event per 1,000 pregnancies. Routine thromboprophylaxis in all pregnant women is therefore not justified. However, women who have previously had a venous thromboembolic event are at a substantially higher risk of recurrent thrombosis and should be considered for thromboprophylaxis in all subsequent high-risk situations, including pregnancy.

Next Article:

A young pregnant woman with shortness of breath

Related Articles

  • From the Editor

    The battle of the clot

    In this issue we review two situations in which low-molecular-weight heparins have special advantages in pregnant women and in patients with...