Research report: Elevated uterine natural killer cell density is associated with severity of recurrent implantation failure in frozen embryo transfer cycles

The research focused on uterine natural killer cells, a population of immune cells that accumulates in the endometrium during the window when an embryo would normally implant, and asked whether their abundance tracks with the severity of recurrent implantation failure.

The study enrolled 94 women undergoing frozen-thawed embryo transfer cycles, dividing them into three groups by the number of failed transfers: 24 women who had failed to conceive after two transfers, 34 after three transfers, and 36 after four or more. Rather than comparing these patients with fertile controls, the investigators deliberately examined the association between uterine natural killer cell density and the number of failed cycles, treating that count as a measure of how severe each woman’s implantation failure had become.

A key strength of the design lies in the timing and standardization of the tissue sampling. All endometrial biopsies were collected on the putative day of blastocyst transfer during a mock, non-conception hormone replacement therapy cycle, specifically five days after the start of progesterone supplementation, a time point labeled P + 5.

To visualize and quantify the cells, the researchers applied immunohistochemical staining for CD56, a surface molecule that identifies uterine natural killer cells, to the endometrial tissue sections. Cell counting followed a standardized protocol, and results were expressed as the percentage of CD56-positive uterine natural killer cells relative to the total number of stromal cells in the tissue, providing a density measure rather than a simple count.

The results revealed a clear pattern across the severity groups. The median uterine natural killer cell density was 1.8 percent, with an interquartile range of 1.4 to 3.3 percent, among women with two failed transfers, and 1.8 percent, ranging from 1.4 to 3.6 percent, among those with three failures. In striking contrast, women with four or more failed transfers had a median density of 3.9 percent, with an interquartile range of 1.9 to 5.8 percent, roughly double the median of the less severely affected groups.

Statistical comparison confirmed that the median density in the group with four or more failures was significantly higher than in the groups with two or three failures. The authors conclude that uterine natural killer cell density was significantly elevated specifically in this high-frequency failure group, highlighting a notable increase in women whose implantation failures were most numerous.

The findings carry several important caveats. Because the study is retrospective and observational, it establishes an association between cell density and the number of failed transfers, not a causal relationship; elevated uterine natural killer cells may be a marker of an underlying endometrial or embryo-related problem rather than a cause of implantation failure itself. The authors also emphasize that the study was not designed to establish a diagnostic threshold for uterine natural killer cell density compared with fertile women, so the results cannot be read as defining a cutoff above which treatment should be considered.

Within those limits, the work adds a graded dimension to a long-running debate in reproductive immunology. Previous studies had already reported increased numbers of uterine natural killer cells in women with recurrent implantation failure relative to fertile control groups, but by stratifying patients according to how many frozen embryo transfers had failed, this study suggests that immune cell density in the endometrium rises in step with the severity of the condition.

Subject of Research: The association between uterine natural killer cell density in the endometrium and the severity of recurrent implantation failure in frozen embryo transfer cycles

Article Title: Elevated uterine natural killer cell density is associated with severity of recurrent implantation failure in frozen embryo transfer cycles

Article References: Yang, W.-J., Zhang, T., Yu, S., Zhao, Y., Li, T. C., & Hsiao, S.-Y. (2026). Elevated uterine natural killer cell density is associated with severity of recurrent implantation failure in frozen embryo transfer cycles. BMC Immunology. https://doi.org/10.1186/s12865-026-00907-8

Image Credits: AI Generated

DOI: 10.1186/s12865-026-00907-8

Keywords: uterine natural killer cells, recurrent implantation failure, frozen embryo transfer, endometrium, CD56, immunohistochemistry, reproductive immunology, retrospective cohort study, BMC Immunology, infertility, Elevated, uterine