The lining of the uterus is called the endometrium. It’s the innermost of three tissue layers that make up the uterine wall, and it plays a central role in menstruation, pregnancy, and reproductive health throughout your life.
How the Endometrium Fits Into the Uterine Wall
The uterus is built from three distinct layers. The perimetrium is the thin, protective outer shell made of epithelial cells. The myometrium is the thick, muscular middle layer that expands during pregnancy and contracts during labor to push a baby out. The endometrium lines the inside, directly facing the uterine cavity. It’s the layer that thickens each month, sheds during a period, and serves as the landing site for a fertilized egg.
The Two Layers of the Endometrium
The endometrium itself has two main zones. The deeper layer, called the basalis, sits closest to the muscle wall and stays permanently in place. It acts as a regenerative base, supplied by its own set of straight blood vessels that keep it nourished even during menstruation. The upper layer, called the functionalis, is the one that changes dramatically throughout the menstrual cycle. It grows, thickens, and is ultimately shed if pregnancy doesn’t occur. The functionalis contains specialized spiral-shaped blood vessels that play a key role in triggering a period when hormone levels drop.
How the Lining Changes Each Month
The endometrium goes through three phases during each menstrual cycle, driven almost entirely by two ovarian hormones: estrogen and progesterone.
During the proliferative phase (roughly the first half of the cycle, after a period ends), estrogen dominates. It stimulates rapid growth of endometrial tissue and blood vessels, rebuilding the functionalis layer from scratch. The lining thickens steadily during this window.
After ovulation, the cycle shifts into the secretory phase. Progesterone takes the lead, transforming the now-thickened lining into a receptive environment for a potential embryo. Stromal cells in the endometrium begin a process called decidualization, where they change into specialized secretory cells that can nourish an implanting embryo and support early placental development. In humans, this transformation actually begins before an embryo arrives, triggered by the natural rise in progesterone after ovulation.
If no pregnancy occurs, the corpus luteum in the ovary breaks down, and both estrogen and progesterone levels drop sharply. This withdrawal triggers a cascade of events in the functionalis layer: the tissue swells, inflammatory signals increase, and the spiral blood vessels constrict tightly. That constriction cuts off blood flow to the upper layer, causing the tissue to break down. The vessels then rupture, and the resulting blood flow dislodges the degenerating tissue. That’s a menstrual period. The basalis layer, fed by its separate blood supply, remains intact and immediately begins regenerating a new functionalis for the next cycle.
The Endometrium’s Role in Pregnancy
When a fertilized egg does reach the uterus, the endometrium is what makes implantation possible. The decidualized lining provides both a physical anchor and a nourishing microenvironment for the embryo. Research published in eLife has shown that signals from the embryo itself actively enhance this process. The embryo releases a signaling molecule that triggers a chain reaction in the uterine lining, promoting the release of compounds that further stimulate decidualization. This two-way communication between embryo and endometrium is essential for successful implantation and the early stages of placental formation.
Once pregnancy is established, the endometrium doesn’t shed. Sustained progesterone production keeps the lining thick and supportive, which is why periods stop during pregnancy.
How Thickness Changes With Age
During the reproductive years, endometrial thickness fluctuates with the menstrual cycle, growing thinner right after a period and reaching its peak thickness during the secretory phase. After menopause, when estrogen and progesterone production drops permanently, the endometrium thins and stays thin. A measurement of 4 mm or less on ultrasound is typical for postmenopausal individuals and carries a greater than 99% negative predictive value for endometrial cancer, according to the American College of Obstetricians and Gynecologists. If postmenopausal bleeding occurs alongside a thicker measurement, further evaluation is usually recommended. Persistent or recurrent bleeding warrants investigation regardless of thickness, since rare forms of endometrial cancer can develop even when the lining appears thin.
Common Conditions That Affect the Endometrium
Several conditions involve abnormal changes to the uterine lining. Endometrial hyperplasia occurs when the cells of the lining grow too thick and begin to crowd together. It’s typically driven by excess estrogen exposure without enough progesterone to balance it. The most common symptom is abnormal uterine bleeding: periods that are heavier or longer than usual, cycles shorter than 21 days, or any bleeding after menopause. Hyperplasia exists on a spectrum. Benign hyperplasia involves overgrowth without dangerous cell changes. Endometrial intraepithelial neoplasia is a precancerous stage where cells have begun to look abnormal. Left unaddressed, hyperplasia can progress to endometrial cancer.
Endometriosis is a different condition entirely. It occurs when tissue similar to the endometrium grows outside the uterus, on organs like the ovaries, fallopian tubes, or pelvic lining. This misplaced tissue still responds to hormonal signals, thickening and breaking down each cycle, but it has no way to exit the body. The result is often chronic pain, inflammation, and sometimes fertility problems.
Endometrial polyps are another common issue: small, usually benign growths that project from the lining into the uterine cavity. They can cause irregular bleeding or, in some cases, interfere with fertility.