In day 25 and 26 endometrium, predecidua forms underneath the surface epithelium and becomes fairly prominent. Note the admixture of predecidualized stroma (right lower image) and edematous non-predecidualized stroma (center image).
Predecidualized stromal cells have abundant pink cytoplasm and plumper nuclei compared to non-predecidualized stromal cells.
Serpiginous glands are embedded in a stroma with increasing predecidualization. Note that unless predecidua formation is advanced, it may be difficult to detect. Thus, one may need to search for areas of unambiguous predecidualization. Dating is based on the most advanced changes.
There are still foci of stroma uninvolved by predecidualization. The stroma is edematous with so-called naked nuclei. Other areas of this endometrium demonstrated more advanced dating features.
In late secretory endometrium (day 23-27), the presence of predecidualization and its location is key to dating.
1 Mazur MT, Kurman RJ. Diagnosis of Endometrial Biopsies and Curettings. New York, NY: Springer; 2005: 12-19.
2 Nucci MR, Oliva Esther. Gynecologic Pathology: Foundations in Diagnostic Pathology. Philadelphia, PA: Elsevier: 2009: 202-4.