In the early 1980s, Turpie and colleagues8 performed a randomized trial in patients undergoing elective hip surgery in North American centers; patients were randomized to receive LMWH bid (enoxaparin 30 mg bid) or placebo with the first dose of study drug given 12 to 24 h after surgery (Table 1).8 The rationale for starting enoxaparin postoperatively was based on three considerations. First, Bergqvist and colleagues6,7 had shown that an early preoperative dose caused excessive bleeding during and soon after surgery. Second, North American orthopedic surgeons were particularly concerned about mitigating peri-operative bleeding complications. Third, VKAs started postoperatively were shown to be effective and safe for VTE prophylaxis despite their delayed anticoagulant effect.15 The study by Turpie and colleagues8 showed that postoperative initiation of enoxaparin at a dose of 30 mg bid was highly effective at reducing the risk of DVT (enoxaparin vs placebo: 10.8% vs 51.3%, P = .0002) without increasing the risk of major bleeding (enoxaparin vs placebo: 4.0% vs 4.0%, P value not reported).8 Two subsequent randomized trials compared enoxaparin 30 mg bid and 40 mg OD with enoxaparin 10 mg OD16 or LDUH 5,000 units tid17 within 24 h after elective hip replacement surgery. The first study showed that enoxaparin 30 mg bid (11% vs 25%, P < .001) and 40 mg OD (14% vs 25%, P = .02) were more effective than enoxaparin 10 mg OD at preventing VTE.16 A comparison between enoxaparin 30 mg bid and 40 mg OD demonstrated similar rates of VTE (11% vs 14%, P > .2) and bleeding (13% vs 11%, P value not reported). The second study showed that enoxaparin 30 mg bid was more effective than both LDUH 5,000 units bid (6% vs 15%, P = .03) and enoxaparin 40 mg OD (6% vs 21%, P = .0003) at preventing VTE. Rates of bleeding were similar with enoxaparin 30 mg bid and both LDUH 5,000 units bid (12% vs 12%, P value not reported) and enoxaparin 40 mg OD (10% vs 12%, P value not reported). Based on these results, the enoxaparin 30-mg bid regimen and not the 40-mg OD regimen was initially approved by the US Food and Drug Administration and widely adopted by North American orthopedic surgeons.