Respectively for patients with repeated transplant failure and routine patients, Thawing frozen embryos in advance (18h) to extend the duration of embryonic development or thawing embryos on the day of transfer in the frozen embryo transfer cycle, to analyze which way can improve clinical pregnancy outcomes, is there a significant difference between the two ways or two types of patients?

Although in clinical practice, thawed embryos with appropriate development time will be selected strictly according to the number of days after ovulation, there are still two problems in actual clinical treatment: one is the embryonic factor, although the rate of embryonic development is generally the same, However, the phenomenon of delayed embryo division and development often occurs. The second is the endometrial factor. There are reports that a small number of people will move the transplant window forward, or the transplant window is narrow. This potentially creates the risk of embryo-endometrial mismatch. Based on this, the research team focused on patients with frozen embryo transfer cycles and put forward the strategy of "thawing embryos in advance and prolonging the development period of embryos properly" in an effort to achieve better repair of damage caused by embryo freezing and fine-tuning of embryo-endometrial interaction time. Therefore, it is necessary to compare thawing embryos in advance and thawing on the day of transfer on pregnancy outcomes in patients with repeated transplant failure and routine patients respectively.

The purpose of this study is to analyze which way can improve clinical pregnancy and birth outcomes, is there a significant difference between the two ways or two types of patients?

Inclusion Criteria:

• Aged <=35 years old;
• Regular menstrual cycle;
• Baseline follicle stimulating hormone (FSH) <12 IU/L;
• Infertility, pure fallopian tube or/and male factor;
• At least 2 high-quality day 3 embryos or 1 high-quality day 5 blastocyst are used for ET.

Exclusion Criteria:

• Patients with repeated transplant failure:
• Abnormal uterus, such as adenomyosis, endometriosis, endometrial polyps or Asherman syndrome;
• Abnormal ovaries, such as low ovarian response and polycystic ovary syndrome (PCOS);
• The endometrial thickness of FET day is <7 mm.

Women in childbearing age intend to do IVF-FET