The purpose of this prospective preliminary clinical study was to assess the efficacy of thromboprophylaxis throughout pregnancy in women with a history of unexplained first trimester miscarriages. From the 53 patients originally assigned to the study 15 were excluded. The remaining 38 were treated with low molecular weight heparin (LMWH-natroparine calcium 0.3 mL twice daily) and low dose aspirin from the day the fetal heart motion was detected until the 37th week or earlier at the onset of premature labor. Among the patients treated (n = 38) thrombophilia screening was positive in I 6 patients and in the remaining 22 no causative factor was detected. The overall success rate (viable pregnancy ≥ 24 weeks) was 92.2% with no significant difference between patients with positive or negative thrombophy a screening. The most significant complications were: preeclampsia (21 %), IUGR (26%), placenta abruptio (5.2%), injection site haematoma (44%) and skin reaction (15.7%). No abnormal bleeding was observed during vaginal or caesarean section. The results of this study suggest that thromboprophylaxis during pregnancy, which has already been successfully tried in patients with recurrent miscarriages with a causative factor, may be similarly effective in patients with such a pregnancy complication but of unknown aetiology.