The uterus consists of three layers—the serosa, the myometrium, and the endometrium—each with a distinct function. This article focuses on the endometrium, the inner lining of the uterus, which plays a key role in conception and pregnancy.

The serosa is the outer layer, producing fluid that reduces friction with nearby organs. The myometrium, composed of smooth muscle, expands during pregnancy and contracts during labor.

The endometrium undergoes monthly hormonal changes. It thickens to receive the blastocyst and, if implantation does not occur, it sheds during menstruation. It consists of three layers: basal, spongy, and compact. The functional layer (spongy + compact) goes through two major phases:

Proliferative Phase

Beginning on the first day of menstruation and lasting until ovulation, this phase is driven by estrogen, which rebuilds and thickens the endometrium. Abnormal estrogen levels can result in a lining that is too thin or too thick.

Secretory Phase

After ovulation, progesterone triggers the secretion of proteins, lipids, and glycogen to nourish a potential embryo. If implantation occurs, hCG maintains progesterone production. If not, progesterone levels drop and the functional layer is shed.

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Endometrial Thickness — Why It Matters

An endometrium that is too thin or too thick can reduce implantation rates and increase the risk of miscarriage. A thin lining may contribute to infertility, and long-term use of oral contraceptives has been associated with temporary endometrial thinning.

What Is Endometriosis?

Endometriosis occurs when endometrial-like tissue grows outside the uterus, often affecting the ovaries, fallopian tubes, intestines, or other pelvic organs. It is one of the leading causes of infertility in women.

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the Endometrium

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