Fetal warfarin syndrome is a disorder of the embryo which occurs in a child whose mother took the medication warfarin (brand name: Coumadin) during pregnancy. Resulting abnormalities include low birth weight, slower growth, intellectual disability, deafness, small head size, and malformed bones, cartilage, and joints.

Warfarin is an oral anticoagulant drug (blood thinner) used to reduce blood clots, deep vein thrombosis, and embolism in people with prosthetic heart valves, atrial fibrillation, or those who have had ischemic stroke. Warfarin blocks the action of vitamin K, causing an inhibition of blood clotting factors and the pro-bone-building hormone osteocalcin.

Warfarin is a teratogen which can cross from the mother to the developing fetus. The inhibition of clotting factors can lead to internal bleeding of the fetus while the inhibition of osteocalcin causes lower bone growth. As well as birth defects, warfarin can induce spontaneous abortion or stillbirth. Because of this, warfarin is contraindicated during pregnancy.