Vitamin K
Warfarin’s ability to cause fetal warfarin syndrome "in utero" stems from its ability to limit vitamin K activation. Warfarin binds to and blocks the enzyme Vitamin K epoxide reductase which is usually responsible for activating vitamin K during vitamin K recycling. Vitamin K, once activated, is able to add a carboxylic acid group to glutamate residues of certain proteins which assists in correct protein folding. Without active vitamin K, a fetus exposed to warfarin is unable to produce large quantities of clotting and bone growth factors.