Since it does not cross the placenta, UFH is not teratogenic. However, multiple reports of thrombosed valves with the use of UFH, causing maternal morbidity and mortality, have raised serious concerns about its effectiveness.2,13- 16 One plausible explanation for the failure of UFH is inadequate dosage. It is known that low-dose heparin (5000 IU every 8-12 hours subcutaneously) is inadequate for prevention of thrombosis of mechanical prosthetic heart valves during pregnancy16; it is unclear whether 12-hourly subcutaneous UFH adjusted to prolong a mid-interval activated partial thromboplastin time (aPTT) result to 1.5 to 2.5 times control is adequate. After initial heparin therapy, this regimen has been shown to be as effective as warfarin #1with a target international normalized ratio [INR] of 2.0-3.0) for the prevention of recurrence in nonpregnant patients with acute venous thromboembolism.17 However, based on several considerations, this regimen of UFH might be less effective in pregnant women with mechanical heart valves. First, 1.5 times control, the usual lower limit of the therapeutic range, corresponds to subtherapeutic heparin levels (anti-factor Xa levels <0.3 U/mL) using most currently available aPTT reagents.18 Second, except for patients who have bileaflet mechanical aortic valves and do not have atrial fibrillation, the recommended target INR range for patients with mechanical heart valves (2.5-3.5) is higher than the corresponding target INR range for the treatment of acute venous thromboembolism (2.0-3.0), suggesting that more intense antithrombotic therapy is appropriate.19 Further, although warfarin, with a target INR of 2.0 to 3.0, is highly effective in the long-term treatment of acute venous thromboembolism, even with the use of a more intense warfarin regimen (INR of 2.5-3.5), the addition of aspirin to the warfarin regimen improves efficacy for patients with mechanical heart valves (albeit at the cost of an increase in the rate of minor bleeding).20 Given this information, we recommend that if subcutaneous UFH is used to prevent thrombosis in pregnant women with mechanical heart valves, the starting dose should be high (17 500-20 000 U every 12 hours) and adjusted aggressively to achieve a mid-interval aPTT of at least 2.0 times control or a result that corresponds to an anti-factor Xa heparin level of at least 0.3 to 0.5 U/mL. Finally, adjunctive aspirin therapy should be considered in high-risk women, such as those with previous systemic embolism and those with atrial fibrillation.