The rate of warfarin embryopathy is reduced (<3%) but not eliminated if the daily dose of warfarin is 5 mg/d.3,6.16,18,23,25,36,37 In most women, the effective dose of warfarin typically does not vary significantly during pregnancy.38 For women who require a dose 5 mg/d, continua- tion of low-dose warfarin throughout pregnancy poses the lowest combined risk to mother and The number of reported pregnancies on low-dose warfarin is relatively small, and not all publications have found improved fetal out- comes on low-dose warfarin,11 so caution should be exercised until more data are available. 9. If warfarin is taken in doses >5 mg/d during the first trimester of pregnancy, there is a >30% risk of fetal loss or embryopathy. For women who require >5 mg/d to maintain a therapeutic INR, replac- ing warfarin with dose-adjusted LWMH during the first trimester reduces fetal loss.3,6,10,15,16,23,39,40 While the patient is taking LMWH, anti-Xa levels should be monitored at least weekly and the dose adjusted accordingly. Fixed dosing is never appro- priate, because it is associated with high maternal morbidity and mortality.41 After the first trimester, the fetal toxicity of warfarin is substantially lower, so switching back to warfarin for the second and third trimesters results in a reasonable balance between maternal safety and fetal safety. 10. In regions where LMWH is unavailable or cost-pro- hibitive, or if anti-Xa levels cannot be monitored, continuous infusion of UFH can be used as an alternative to LMWH during the first trimester for women who require a warfarin dose of >5 mg/d.37 If UFH is used during the first trimester, the dose should be adjusted to maintain an aPTT 2 times control. 