Figure 1. Hematoxylin-eosin staining for histological analysis of the endometrium on days 16-24 of the menstrual cycle, with key features under Noyes criteria. Key features: Day 16: Subnuclear vacuolation of the glandular epithelium is regular and becomes prominent, pseudostratification of the nuclei is still visible in the glandular epithelium, and stroma is dense with nearly naked nuclei. Day 17: Only focal stratification of the glandular epithelium is present, nuclei are more or less in a row, with homogeneous cytoplasm above them and large vacuoles below. Day 18: There is almost no stratification of the glandular epithelium. Vacuoles have decreased in size and slip past the nucleus into the cytoplasm nearer the glandular lumen and thence into the lumen. Day 19: Few vacuoles remain. Superficially, this phase may resemble the early vacuolation of the 16-day phase. The presence of intraluminal secretion and absence of pseudostratification and of mitoses serve to characterize it. Day 20: Secretion reaches its peak. Subnuclear vacuoles are rare and the nuclei are round and basally located. Secretion, positively stained with periodic acid-Schiff reaction (red colour), becomes prominent and is visible until day 25. Start of stromal edema. Day 21: Stromal edema increases. Day 22: Stromal edema reaches its peak. The spiral arterioles are not prominent, and stromal cells are nearly naked nuclei, with only thin cytoplasm. Day 23: Spiral arterioles become prominent due to condensation of their surrounding stroma. The nuclei and the cytoplasm of the periarteriolar stromal cells become larger. This is the earliest predecidual reaction. Day 24: Spiral arterioles become prominent due to the pre-decidual reaction around them. Pre-decidual changes start to differentiate focally under the surface epithelium. This is normally expected at day 25 by Noyes criteria, but in the present study it was seen irregularly in combination with pre-decidual reactions around spiral arterioles, suggesting that it starts on day 24. Stromal proliferation resumes. Magnification: Main image ×100; inset: ×400.