Heparin has been assigned to pregnancy category C by the FDA. Animal studies have not been conducted and there are no controlled data in human pregnancy. It does not cross the placenta due to its large molecular weight (3,000 to 30,000 daltons). Heparin is considered the anticoagulant of choice during pregnancy, although anticoagulation and maternal disease state may pose a risk to both the mother and fetus. Heparin is only recommended for use during pregnancy when benefit outweighs risk.

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Heparin is not excreted into human milk due to its large molecular weight (3,000 to 30,000 daltons). Heparin is considered compatible with breast-feeding.

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