The aim of this study is to verify the efficacy of in vitro induction of autologous decidual-like NK cells therapy for reproductive failure associated with uterine NK cells abnormalities.

Participants with reproductive failure and reproductive intentions will be recruited and screened. After obtaining their informed consent, menstrual blood analysis will be performed. Participants with menstrual blood NK cell abnormalities will be randomly assigned to either the NK cell perfusion group or control group in a ratio of 2:1. After completing two rounds of NK cell therapy, the NK cell perfusion group will enter the follow-up period, while the control group will not receive interventions after randomization and enter the follow-up period directly. The following information will be recorded for all participants who undergo menstrual blood NK cell analysis, including menstrual blood analysis results, attempts to conceive, and pregnancy outcomes during the follow-up period.

Inclusion Criteria:

• Reproductive failure with abnormal endometrial natural killer (NK) cells in menstrual blood;
• With a clear intention to conceive;
• Normal ovarian function, or premature ovarian insufficiency with ≥2 high-quality cryopreserved embryos available for transfer;
• Endometrial thickness measured by transvaginal ultrasound during the periovulatory period ≥7 mm;
• Body mass index (BMI) between 18 kg/m² and 28 kg/m²;
• Willing to comply with the follow-up plan of this study.

Exclusion Criteria:

• Use of progesterone receptor modulators;
• One of the couple's chromosomal karyotype abnormalities, with no euploid embryo identified via preimplantation genetic testing for aneuploidy;
• Severe endometriosis or adenomyosis, uterine fibroids affecting uterine cavity morphology, uterine malformations, intrauterine adhesions, or thin endometrium;
• Uncontrolled autoimmune or endocrine disorders;
• Contraindications to pregnancy;
• History of malignant tumors;
• Participating in other clinical studies;
• Allergy to blood products.