摘要：Objective The purpos. of this.s.udy was.to conduct pilot s.udies.for large randomized controlled trials.to compare low molecular weight heparin with placebo for antenatal thromboprophylaxis.(trial 1), and for thromboprophylaxis.after ces.rean delivery (trial 2). s.udy des.gn Multicenter randomized controlled trials.(trial 1, 23 units. trial 2, 8 units. were conducted. Pregnant women at increas.d ris. for thromboembolic dis.as. were eligible for trial 1; women who underwent ces.rean 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 s.mptomatic thromboembolic events.and s.mptomatic os.eoporotic fractures. trial 2, confirmed s.mptomatic thromboembolic events.and wound complications. Res.lts.s.xteen women were recruited for trial 1; 1 woman in the placebo group had a s.mptomatic thromboembolic event. One hundred forty one women were recruited for trial 2; 1 woman in the low molecular weight heparin group had a s.mptomatic thromboembolic event. Conclus.on Poor recruitment indicates.that large s.ale trials.us.ng thes. des.gns.would be difficult. Collection of data on the number of women that are eligible and the reas.ns.for nonrecruitment in future trials.of thes. interventions.would allow a better unders.anding of the reas.ns.for poor recruitment.