The terminal half-life is 17 h in young subjects and 21 h in elderly volunteers. Fondaparinux produces a predict- able anticoagulant response and exhibits linear phar- macokinetics when given in subcutaneous doses of 2 to 8 mg or in IV doses ranging from 2 to 20 mg.200 There is minimal nonspeci c binding of fondaparinux produced a dose-dependent decrease in cancellous bone volume in rats. However, the effects were greater with UFH than with LMWH.185 These inves- tigators also showed that although both anticoagu- lants inhibited bone nodule formation and increased alkaline phosphatase in a dose-dependent manner, UFH had a sixfold greater effect than LMWH.30 Other investigators also reported that LMWH causes signi cant inhibition of osteoblast growth186 and pro- duces osteopenic changes in rats.187 Three small prospective clinical studies have reported on the effects of prophylactic doses of LMWH on bone density. The rst was a cohort study in which 16 women receiving enoxaparin (40 mg daily) during pregnancy had serial bone density measurements of the proximal femur. Baseline mea- surements were taken within 2 weeks of starting therapy and then at 6 to 8 weeks and 6 months post- partum. Patients received enoxaparin for a mean duration of 25 weeks (range, 19-32 weeks). Com- pared with baseline values, there was no signi cant change in mean bone density at 6 weeks postpartum and no patient experienced a . 10% decrease in bone mass. At 6 months postpartum, there was a sig- ni cant reduction in mean bone density (P 5 .02) and two of the 14 patients evaluated (14%) had The second study was an open randomized trial that included 44 pregnant women with VTE. 