Recurrent implantation failure (RIF) is considered a tough problem in assisted reproductive technology (ART) without effective treatments. The effect of low molecular weight heparin (LMWH) in pregnancy outcomes is controversial. In addition, there are few reports on the role of LMWH in population with RIF performing freeze-thaw embryo transfer (FET) cycles. The purpose of this study is to evaluate the effect of LMWH on pregnancy outcomes in women with FET cycles.

Inclusion Criteria:

• Women under or equal to the age of 40 when oocytes were retrieved;
• The history of at least 2 failed fresh embryo transfer/FET cycles and cumulative transfer of ≥ 3 high-quality embryos or cumulative transfer of ≥ 2 blastocysts;
• Planning to FET after IVF or ICSI cycles

Exclusion Criteria:

• Women with untreated hydrosalpinx, uterine cavity occupying lesions (uterine adhesions, submucous fibroids, endometrial polyps, et al.);
• Women with abnormal anatomical structure of uterus (unicornuate uterus, bipedal uterus, et al.);
• Women with endometrial thickness (EMT) < 6mm on the day of transplantation;
• Women or their partner with abnormal chromosome karyotype (not including chromosome polymorphisms);
• Women with the history of recurrent spontaneous abortion (RSA);
• Women with a history of autoimmune or coagulation disorders;
• Women with contraindications to LMWH

The study analyzed the data from the hospital electronic database for patients undergoing FET between January, 2021 and November, 2024, a total of 392 transplant cycles, who undergoing at least two IVF or ICSI cycles from the Second Hospital of Shandong University.

The investigators fully acknowledge the significance of data sharing as stipulated by SCI journals. However, owing to the policies and confidentiality agreements adhered to in our laboratory, the investigators regretfully cannot furnish the raw data. Nevertheless, the investigators have meticulously presented a comprehensive account of the experimental design, analysis, results, and the procedures employed for data analysis and processing.