Warfarin use during pregnancy is not recommended because it may cause birth defects. In addition, use during the last six months of pregnancy may cause severe bleeding in the fetus and, in the last two weeks of pregnancy, may cause bleeding in both the fetus and the mother. However, heparin is routinely prescribed for pregnant women who have mechanical valves.

Some anticoagulants pass into nursing babies through breast milk. Physicians may monitor nursing infants to detect any unwanted effects or prescribe a medication that would overcome those effects. Patients should consult their physician about breast-feeding when taking anticoagulants.