<P> Warfarin administration in the second and third trimesters is much less commonly associated with birth defects, and when they do occur, are considerably different from fetal warfarin syndrome . The most common congenital abnormalities associated with warfarin use in late pregnancy are central nervous system disorders, including spasticity and seizures, and eye defects . Because of such later pregnancy birth defects, anticoagulation with warfarin poses a problem in pregnant women requiring warfarin for vital indications, such as stroke prevention in those with artificial heart valves . </P> <P> According to the American College of Chest Physicians (ACCP), warfarin may be used in lactating women who wish to breast - feed their infants . Available data does not suggest that warfarin crosses into the breast milk . Similarly, INR levels should be checked to avoid adverse effects . </P> <P> The only common side effect of warfarin is bleeding . The risk of severe bleeding is small but definite (a typically yearly rate of 1 - 3% has been reported) and any benefit needs to outweigh this risk when warfarin is considered . All types of bleeding occur more commonly, but the most severe ones are those involving the brain (intracerebral hemorrhage / hemorrhagic stroke) and the spinal cord . Risk of bleeding is increased if the INR is out of range (due to accidental or deliberate overdose or due to interactions). This risk increases greatly once the INR exceeds 4.5 . </P> <P> A number of risk scores exist to predict bleeding in people using warfarin and similar anticoagulants . A commonly used score (HAS - BLED) includes known predictors of warfarin - related bleeding: uncontrolled high blood pressure (H), abnormal kidney function (A), previous stroke (S), known previous bleeding condition (B), previous labile INR when on anticoagulation (L), elderly as defined by age over 65 (E), and drugs associated with bleeding (e.g. aspirin) or alcohol misuse (D). While their use is recommended in clinical practice guidelines, they are only moderately effective in predicting bleeding risk and do not perform well in predicting hemorrhagic stroke . Bleeding risk may be increased in people on hemodialysis . Another score used to assess bleeding risk on anticoagulation, specifically Warfarin or Coumadin, is the ATRIA score, which uses a weighted additive scale of clinical findings to determine bleeding risk stratification . The risks of bleeding are increased further when warfarin is combined with antiplatelet drugs such as clopidogrel, aspirin, or nonsteroidal anti-inflammatory drugs . </P>

Where does warfarin act in the clotting cascade