Effect of Low Molecular Weight Heparin in Freeze-thaw Embryo Transfer Cycles in Women With Recurrent Implantation Failure

Inclusion Criteria: * Women under or equal to the age of 40 when oocytes were retrieved; * The history of at least 2 failed fresh embryo transfer/FET cycles and cumulative transfer of ≥ 3 high-quality embryos or cumulative transfer of ≥ 2 blastocysts; * Planning to FET after IVF or ICSI cycles Exclusion Criteria: * Women with untreated hydrosalpinx, uterine cavity occupying lesions (uterine adhesions, submucous fibroids, endometrial polyps, et al.); * Women with abnormal anatomical structure of uterus (unicornuate uterus, bipedal uterus, et al.); * Women with endometrial thickness (EMT) \< 6mm on the day of transplantation; * Women or their partner with abnormal chromosome karyotype (not including chromosome polymorphisms); * Women with the history of recurrent spontaneous abortion (RSA); * Women with a history of autoimmune or coagulation disorders; * Women with contraindications to LMWH