By day 27, the precidualization is extensive and it should be easily identified in most endometrial fragments.

Precidual change is prominent, as demonstrated by stromal cells with abundant eosinophilic cytoplasm and plump nuclei. The cytoplasmic borders are indistinct. Precidual is well represented in the right side of this image; uninvolved stromal cells are seen in the center of the image.

Predecidua is seen surrounding arterioles and involves a large portion of the stroma in this image. Note the glands at the top contain luminal secretions. Luminal secretions can be seen throughout mid to late secretory phase.

The glandular morphology in late secretory endometrium can range from glands exhibiting secretory exhaustion, to glands containing abundant luminal secretions, to glands with variable numbers of secretory vacuoles. All three are seen here.

In late secretory endometrium (day 23-27), the presence of predecidualization and its location is key to dating.

• Day 23, the predecidua is located around the individual spiral arterioles.
• Day 24, the predecidua bridges multiple vessels.
• Day 25, the predecidua forms a thin, discontinous band beneath the superficial epithelium.
• Day 26, the predecidua forms a thick continuous band beneath the superficial epithelium.
• Day 27, the predecidua is mostly confluent in the stroma.