Materials & Methods This study was a prospective randomized controlled trial to assess whether administration of unfractionated heparin would increased implantation and pregnancy rates in women with repeated ET-IVF failures or not . The study was performed at a reproduction center affiliated to a medical university. A total 86 patients who were candidate for IVF/ICSI with a history of three or more pervious IVF-ET failures enrolled in this study . The study was approved by the ethics committee of Research and Clinical Center for Infertility affiliated to Yazd Medical University of Medical Sciences . All patients were required to sign a written consent after the provision of complete information to them .