This study is a single-center, randomized, controlled prospective study. Those patients with repeated implantation failure (RIF) who will recieve frozen thawed embryo transfer (FET) are enrolled in the study. To determine the effect of vaginal progesterone on the clinical pregnancy outcomes of RIF patients.

According to the enrollment and exclusion criteria, the patients were enrolled, and the subjects were randomly divided into two groups by computer randomization. Group A was the vaginal progesterone added group (test group). Group B was the regular FET group (control group).

Inclusion Criteria:

1. Aged between 20 and 40 years old;

2. BMI: 18-28 kg/m2；

3. Consistent with the diagnosis of repeated implantation failure of unknown reasons, the previous embryo transfer situation meets one of the following:

   Total embryos transferred ≥ 4 high-quality cleavage-stage embryos; ≥ 2 blastocysts; ≥ 2 high quality cleavage-stage embryos +≥ 1 blastocysts;

4. At least 1 high-quality embryo remained for embryo transfer;

5. Volunteer to participate in the study and sign the informed consent form.

Exclusion Criteria:

1. Patients with recurrent pregnancy loss (≥ 2 biochemical pregnancies or ≥ 2 spontaneous abortions);
2. Adverse pregnancy history (stillbirth, fetal malformation, etc.);
3. Severe paternal factors: need for TESA or PESA;
4. PGT;
5. Failure of embryo implantation due to any definite reason, including but not limited to: endometrial adhesion (moderate to severe), thin endometrium (<7 mm before transformation), endometritis, endometriosis (medium or severe), adenomyosis, untreated hydrosalpinx, hysteromyoma (submucosal fibroids, non submucosal fibroids > 4.0 cm and/or endometrial compression), reproductive malformation, serious immune disease, serious coagulation function abnormality;
6. Chromosome abnormality of either spouse;
7. Those with contraindications to pregnancy or assisted reproductive technology.