Objective The purpose of this study was to conduct pilot studies for large randomized controlled trials to compare low molecular weight heparin with placebo for antenatal thromboprophylaxis (trial 1), and for thromboprophylaxis after cesarean delivery (trial 2). Study design Multicenter randomized controlled trials (trial 1, 23 units; trial 2, 8 units) were conducted. Pregnant women at increased risk for thromboembolic disease were eligible for trial 1; women who underwent cesarean delivery were eligible for trial 2. The interventions were once daily injections of low molecular weight heparin or placebo. Primary outcomes were as follows: trial 1, confirmed symptomatic thromboembolic events and symptomatic osteoporotic fractures; trial 2, confirmed symptomatic thromboembolic events and wound complications. Results Sixteen women were recruited for trial 1; 1 woman in the placebo group had a symptomatic thromboembolic event. One hundred forty one women were recruited for trial 2; 1 woman in the low molecular weight heparin group had a symptomatic thromboembolic event. Conclusion Poor recruitment indicates that large scale trials using these designs would be difficult. Collection of data on the number of women that are eligible and the reasons for nonrecruitment in future trials of these interventions would allow a better understanding of the reasons for poor recruitment. Objective The purpose of this study was to conduct pilot studies for large randomized controlled trials to compare low molecular weight heparin with placebo for antenatal thromboprophylaxis (trial 1), and for thromboprophylaxis after cesarean delivery (trial 2). Study design Multicenter randomized controlled trials ( trial 1, 23 units; trial 2, 8 units) were conducted. Pregnant women at increased risk for thromboembolic disease were eligible for trial 1; women who underwent cesarean delivery were eligible for trial 2. The interventions were once daily injections of low molecular weight Primary outcomes were as follows: trial 1, confirmed symptomatic thromboembolic events and symptomatic osteoporotic fractures; trial 2, confirmed symptomatic thromboembolic events and wound complications. Results Sixteen women were recruited for trial 1; 1 woman in the placebo group had a symptomatic thromboembolic event. One hundred forty one women were recruited for trial 2; 1 woman in the low molecular weight heparin group had a symptomatic thromboembolic event. Conclusion Poor recruitment indicates that large scale trials using these designs would be difficult. Collection of data on the number of women that are eligible and the reasons for nonrecruitment in future trials of these interventions would allow a better understanding of the reasons for poor recruitment.