The effect of unfractionated heparin in assisted reproductive technology (ART) cycles is prevention of thrombosis in implantation site . Although its effect is not restricted to anticoagulation and also can modulate apposition , adhesion , and penetration of embryo . Other advantages are decreasing thrombophilic risk in COH ( controlled ovarian hyperstimulation) cycles with administration of gonadotrophins . So heparin make better pregnancy rate in repeated IVF-ET failures.