the inorganic surface and turbulent blood flow generated by a mechanical prosthesis. The risk of blood clotting is further increased by generalized hypercoagulability as concentrations of clotting factors rise during pregnancy. This increased chance of blood clots leads to an increased risk of potentially fatal pulmonary or systemic emboli cutting off blood flow and oxygen to critical organs. Thus, some patients may continue taking warfarin throughout the pregnancy despite the risks to the developing child. Mechanism 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. Without vitamin K, clotting factors II, VII, IX and X are unable to be produced. Without these vital parts of the coagulation cascade a durable fibrin plug cannot form to block fluid escaping from damaged or permeable vasculature. Anemia is common in fetuses exposed to warfarin as blood constantly seeps into the interstitial fluid or amniotic cavity. High doses of warfarin and heavy bleeding lead to abortion and stillbirth. Osteocalcin is another protein dependent on vitamin K for correct folding and function. Osteocalcin is normally secreted by osteoblast cells and plays a role in aiding correct bone mineralization and bone maturation. In the presence of warfarin and subsequent absence of vitamin K and active osteocalcin, bone mineralization and growth are stunted. Prevention Fetal warfarin syndrome is prevented by withholding prescription to pregnant women or those trying to conceive. As warfarin can remain in the mother's body for up to five days, warfarin should not be administered in the days leading up to conception. Doctors must take care to ensure women of reproductive age are aware of the risks to the baby should they get pregnant, before prescribing warfarin. For some women, such as those with prosthetic heart valves, anticoagulation medication cannot be suspended during pregnancy as the risk of thrombus and emboli is too high. In such cases an alternate anticoagulant, which cannot pass through the placental barrier to the fetus, is proscribed in place of warfarin. Heparin