The early secretory endometrium is a stage of the menstrual cycle in which a nearly mature endometrium has a layer of grandular epithelium with round nuclei, thickened endometrium and curled uterine glands with collections of glycogen within them. This stage typically happens two days after ovulation.
Prominent subnuclear vacuoles serve cells in the endometrium over the next 14 days post-ovulation. The presence of these vacuoles can be significant when doing a histologic study to determine the age of the
endometrium. Epithelial cells with round nuclei are formed at the apical pole of the uterine glands within the developing endometrium. Production and storage of glycogen occurs at the basal pole of uterine glands. The endometrium differentiates itself beginning in the early secretory phase, beginning its development due to the hormone progesterone from the corpus luteum.
The glycogen moves away from the basal pole towards the apical pole, shifting the nuclei of the epithelial cells and glycogen releases into the grandular lumen. As these changes occur, the endometrium matures and moves in the middle secretory phase. The period from the 20th to 23rd day of the menstrual cycle, when this endometrium is mature, is considered the time of maximal reception ability. This time-frame of fertility, also known as the implantation window, holds the best chance for a blastocyst to implant and develop into a fetus.