The most widely used oral anticoagulants are coumarin derivatives. During pregnancy, these agents are toxic to the developing fetus. The incidence of fetal damage is related to the stage of gestation. Exposure during the first trimester results in an incidence of fetal warfarin syndrome of up to 25 percent. Later in pregnancy, an estimated 5 percent of infants have cerebral hemorrhage with potential neurologic sequelae. Despite the paucity of evidence, many physicians believe that coumarinderived compounds should not be used during lactation. A review by Clark and colleagues stresses that some anticoagulants are not excreted into breast milk and can be used safely during lactation. In this situation, oral agents offer better compliance and lower cost, and may be more effective than heparin therapy.

The various oral anticoagulants are all small organic molecules, but they differ significantly in polarity. The less polar molecules are more lipophilic and more likely to be excreted in breast milk. Less polar compounds, such as phenindione and anisindione, produce effects that are dose-dependent. At dosages higher than 50 mg, detectable milk levels are almost always achieved. In spite of this finding, only one case of bleeding in an infant (a scrotal hematoma) has been reported related to use of this class of oral anticoagulants by a breast-feeding mother.

Warfarin is highly polar, nonlipophilic and highly bound to plasma proteins, especially albumin. In two small case series, no warfarin was detected in the milk of patients taking up to 12 mg daily. No warfarin could be detected in the blood of the breast-fed infants, and they had no adverse bleeding effects.

The authors conclude that warfarin does not appear to be excreted in breast milk. Because the risk of venous thrombosis is high in women requiring therapeutic anticoagulation postpartum, they advocate use of warfarin when indicated. Although no studies have addressed the issue, the authors recommend performing coagulation studies in infants at risk of vitamin K deficiency. Rarely, mothers may be unable to tolerate warfarin. In this situation, the authors recommend dicumarol as an alternative anticoagulant.