Day 23 endometrium is defined by predecidua forming around spiral arterioles, as seen here. However, there is no solid bridging of precidua between arterioles, suggesting features closer to day 24.

Although it is quite subtle in this image, predecidua is starting to forming around the arterioles (upper left). Note the prominent stromal edema.

Day 23 endometrium may demonstrate predecidua formation between glands. However, this should be only focal and solid confluence of predecidua between multiple spiral arterioles and glands would be designated as day 24.

Predecidua is forming between glands. Note that the glands are increasing complexity and exhibiting secretory exhaustion. Secretory exhaustion is characterized by glands lined with a single layer of epithelial cells with little or absent secretory vacuoles.

In other areas of the endometrium, predecidualization has not yet occurred. The stroma is edematous. The glands have basally oriented nuclei and secretory vacuoles are largely absent.

The glands in this image have variable amounts of subnuclear and supranuclear secretory vacuoles, which can be seen in late secretory endometrium. Note that predecidua is forming between glands.

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.