Abstract： Background Reduced endometrial receptivity in hyperstimulated cycles may lead to a lower implantation rate and a lower clinical pregnancy rate,but it is unclear if it is also associated with an increase in pregnancy loss rate.The aim of this study was to compare the implantation,miscarriage,and pregnancy rates between fresh and frozen thawed transfer of one or two day-3 embryos,with a view to understanding whether or not reduced endometrial receptivity encountered in hyperstimulated cycles is associated with an increase in miscarriage rate.Methods This study involved a consecutive series of 1 551 single day-3 embryo transfer cycles and consecutive 5 919 double day-3 embryo transfer cycles in the Assisted Reproductive Unit of the Sir Run Run Shaw Hospital,Hangzhou,China,between January 2010 and December 2012.Results The implantation and clinical pregnancy rates (single embryo 30.7％ and double embryos 33.4％ and 51.4％)using fresh cycle were both significantly lower than that of frozen-thawed cycles (single embryo 35.8％ and double embryos 38.1％ and 57.8％).There was no difference in biochemical loss or clinical miscarriage rates between the two groups.Conclusions Impairment of endometrial receptivity associated with ovarian hyperstimulation leads to implantation failure at a very early stage,resulting in an increased number of non-pregnancy.It does not lead to increase in biochemical or clinical losses.The significantly reduced ongoing pregnancy rates in both fresh single and double embryo transfer are therefore due to failure to achieve a pregnancy,rather than pregnancy loss after conception.