Entry criteria were changed, investigator have registrated a new trials.

Repeated implantation failure（RIF） is a insurmountable bottleneck in assisted reproductive technology, many studies have considered that the cause of two-thirds of implant failure is the decreased endometrial receptivity. Human chorionic gonadotropin (hCG) is an early pre-plant signal molecule secreted by the embryo, it can promote endometrial proliferation, increase blood flow and promote embryonic adhesion and inhibit self-regulate apoptosis of trophoblast cells. Previous studies showed that: intrauterine injection of HCG before embryo transfer can improve clinical outcomes in IVF/Intracytoplasmic sperm injection(ICSI). But some studies found that the intrauterine injection of HCG can not significantly improve the success rate of blastocyst transfer, and the reason may be the intrauterine injection of HCG time is too late to significantly increase the implantation rate. Would ahead of intrauterine injection of HCG be more effective? Thus, the patients of repeated implantation frozen embryo cycle according to the random principle accepted two kinds of transplants ways: ①intrauterine injection of HCG before blastocyst transfer; ②blastocyst transfer. Try to understand whether intrauterine injection of HCG can significantly improve the clinical pregnancy rate of blastocyst transfer in repeated implantation failure patients.

Inclusion Criteria:

• women with unexplained RIF(two to four previous embryo transfers without achieving pregnancy);
• age<36 years;
• frozen embryo transfer cycles

Exclusion Criteria:

• polycystic ovary syndrome
• uterine abnormalities (double uterus, bicornuate uterus, unicornuate uterus and uterine mediastinum)
• intrauterine adhesions
• endometriosis
• adenomyosis
• hydrosalpinx
• uterine fibroids (submucosal fibroids, non-mucosal fibroids > 4 cm and / or endometrial pressure)
• thydroid dysfunction and hyperprolactinemia