Dynamic Changes in Endometrial Folding and Secretory Activity Across the Menstrual Cycle

The successful implantation of an embryo remains a significant challenge in assisted reproductive technology, with failure occurring in around 30% of euploid embryo transfers. Implantation demands a synchronized interaction between the blastocyst and the receptive endometrium during the window of implantation (WOI). However, non-invasive methods to study the structural and molecular characteristics of receptivity are scarce.

In this study, researchers examined paired sonohysterogram images and uterine lavage samples gathered during the proliferative and mid-secretory phases from women with regular ovulatory cycles and confirmed fertility. The endometrial folds were measured, while the lavage samples were analyzed using Luminex multiplex immunoassay.

It was found that folds were present in both phases, but they were significantly more abundant during the mid-secretory WOI, regardless of the imaging view or endometrial thickness. The folding was strongly linked to the circulating estradiol level during the proliferative phase but not during the mid-secretory phase. Furthermore, the folding patterns between the phases were not correlated, indicating separate regulatory mechanisms.

The uterine lavage results revealed phase-specific variations in the expression of factors linked to endometrial receptivity and implantation. The glandular epithelium and myeloid-lineage cells stood out as key contributors. In summary, these results suggest that coordinated structural and secretory processes occur during the WOI, and endometrial folding and uterine lavage could serve as complementary, minimally invasive markers of endometrial receptivity.