For these women, dose-adjusted LMWH throughout pregnancy is the safest alterative. LMWH throughout preg- nancy is associated with a higher rate of throm- botic complications than warfarin. However, many of the thrombotic events occur when LMWH is administered improperly or monitored erratically or if patients are nonadherent.8,14,15,39 When administered and monitored meticulously, LMWH can be safe.12 Effective dose monitoring includes weekly measurements of anti factor Xa levels, with additional monitoring after dose Measurement of trough levels to maintain a trough Xa level >0.6 IU/mL may help women maintain therapeutic anticoagulation while on LMWH.12,13 13. When warfarin is taken at a dose 5 mg/d, the risk of warfarin embryopathy is reduced but not entirely eliminated. Some women, after discussion with their physicians, may choose to substitute CLINICAL STATEMENTS AND GUIDELINES LMWH for low-dose warfarin during the first tri- mester to eliminate the risk of warfarin embry- opathy. This choice improves fetal outcomes but at the cost of increased maternal thrombotic 14. Low-dose aspirin is regarded as safe during preg- nancy and can be continued in women with mechanical heart valves if needed for other indi- cations. There may be noncardiac indications for aspirin in pregnant women, such as prevention of 15. Studies using subcutaneous LMWH at a fixed dose without monitoring of anti-Xa levels in preg- nant patients with mechanical prostheses found a high risk of valve thrombosis and maternal death. In pregnant women treated with dose-adjusted LMWH, the dose of LMWH required to maintain an adequate anti-Xa level 4 to 6 hours after admin- istration increases throughout pregnancy.9,15,39,41 16. 