Repeated implantation failure（RIF） is a insurmountable bottleneck in assisted reproductive technology, many studies have considered that the cause of two-thirds of implantation failure is the decreased endometrial receptivity. After exclude some major local immune factors(NK,CD138 cells) and implantation window out of phase, There are still a part of patients infertile .Human chorionic gonadotropin (hCG) is an early pre-implantation signal molecule secreted by the embryo, it can promote endometrial proliferation, increase blood flow and promote embryonic adhesion and inhibit self-regulated 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:

• repeated implantation failure patients: ≥ 3 times IVF-embryo transfer failed to help pregnancy (biochemical or non-pregnant)
• age ≤36 years old
• normal histopathological stage (+5-7)
• endometrial NK cell <4.5%
• 0 endometrial CD138 positive cell
• natural cycle frozen embryo transfer
• frozen blastocysts (≥4BC) embryos ≥ 1

Exclusion Criteria:

• Scar uterus (diverticulum or incision false lumen after cesarean section)
• intrauterine adhesions
• untreated hydrosalpinx
• adenomyosis (endometrial displacement)
• endometritis
• uterine fibroids compress the endometrium