The Endometrium: An Overview

Written by Stephanie Watson
Medically Reviewed by Nivin Todd, MD on February 28, 2025
6 min read

The endometrium is the layer of tissue lining the inside of the uterus – the reproductive organ in the pelvis where a baby develops. "The endometrium is a crucial part of the female reproductive system and plays a key role in the menstrual cycle and pregnancy," says Gil Weiss, MD, an OB/GYN at the Association for Women's Health Care in Chicago. 

Each month, the endometrium sheds and then rebuilds itself during the menstrual cycle. The hormones estrogen and progesterone control this process.

Layers of endometrium

Below the endometrium is the myometrium. It's the muscle layer of the uterine wall that helps the uterus contract during labor. The endometrium is made up of two layers. Right above the myometrium is the stratum basalis, or basal layer. Above that is the functional layer, or stratum functionalis.

Stratum basalis

The stratum basalis is the bottom layer of the endometrium that is attached to the myometrium. During a menstrual period, the stratum basalis stays in place and doesn't shed. It helps regrow the endometrium in preparation for the next menstrual cycle.

Stratum functionalis

This is the thicker outer layer of the endometrium. It changes in response to hormones. During each menstrual cycle, estrogen makes the stratum functionalis grow. If an egg isn't fertilized, progesterone causes this layer to shed, along with blood, during a menstrual period.

The thickness of the endometrium varies at different parts of the menstrual cycle. The lining is thinnest during the first stage of the 28-day cycle, when you get your period. It's thickest during the last stage of the cycle when it's preparing for a fertilized egg to implant.

This is how the size of your endometrium changes during your menstrual cycle:

Menstrual phase: Days 1 to 5. This phase starts on the first day of your period. The endometrium is thinnest because this is when it sheds. It measures 1 to 4 millimeters (mm).

Proliferative phase: Days 5 to 14. During this phase, the endometrium grows and thickens to prepare for a possible pregnancy. It measures 4 to 8 mm.

Secretory phase: Days 15 to 28. The endometrium is thickest during this final phase of the menstrual cycle. It measures up to 14 to 18 mm.

Hormones control endometrial thickness. When estrogen levels drop during menopause, the endometrium gets thinner. The average thickness after menopause is less than 5 mm. 

A thicker uterine lining after menopause could mean you have a problem like uterine polyps, an infection, cysts, or endometrial cancer. "If the endometrium is thicker than 5 mm, it might require investigation by your health care provider," Weiss says.

The endometrium has three basic jobs: 

  • It builds up and thickens to prepare for a fertilized egg to implant.
  • It maintains the pregnancy if a fertilized egg does implant.
  • It sheds the uterine lining if there is no pregnancy.

Each month, the endometrium gets thicker and fills with blood to prepare for a possible pregnancy. If an egg is released from the ovary and fertilized, the developing embryo will implant in the uterine lining and grow. If the egg isn't fertilized, the hormone progesterone is released and the endometrial lining sheds during your period.

A few different conditions affect the lining of the uterus, including these:

Adenomyosis. This is when the endometrium grows into the myometrium, the inner wall of the uterus. It still grows and bleeds like it would in the uterus, which causes pelvic pain and heavy bleeding during periods.

Endometriosis. Tissue that looks and acts like the endometrium grows in other parts of the pelvis, like the ovaries or fallopian tubes.

Endometritis. This is when the endometrium becomes inflamed, usually due to an infection, or after surgery or childbirth. Antibiotics can help clear up the infection. 

Endometrial hyperplasia. In this condition, the endometrium is thicker than usual. Hyperplasia can sometimes grow into endometrial cancer.

Uterine fibroids. These very common noncancerous tumors grow underneath the endometrium. Fibroids cause symptoms like heavy bleeding and pelvic pressure. Left untreated, they can make it harder to get pregnant. 

Uterine polyps. These grow from the inner wall of the uterus and are attached by little stalks. Polyps can be as tiny as sesame seeds or as large as golf balls, and you can have more than one of them. Though polyps are usually benign, they can be precancerous or cancerous, so they should be checked out by your doctor. These polyps can cause heavy or irregular periods, bleeding between periods, and infertility.

Endometrial cancer. This is a type of cancer that grows from the lining of the uterus.

Endometriosis

If you have this condition, tissue that's similar to the lining of your uterus grows in other parts of your pelvis. Common places to find endometriosis are: 

  • The space behind the uterus
  • Inside or on the ovaries and tubes
  • In the myometrium
  • In the fallopian tubes

The tissue still acts like it would inside the uterus. Each month during the menstrual cycle when the endometrium expands in the uterus, it also expands in other parts of the pelvis. This growth causes symptoms like painful cramps and heavy bleeding.

Other symptoms of endometriosis are:

  • Pain during or after your period
  • Spotting between periods
  • Painful sex
  • Bloating
  • Pain when you poop or pee
  • Diarrhea or constipation
  • Depression or anxiety
  • Tiredness
  • Infertility – trouble getting pregnant

Treatments for endometriosis include hormonal medicines like birth control pills, gonadotropin-releasing hormone (GnRH) antagonists, or danazol (Danocrine). These medications prevent symptoms by making your periods lighter or stopping them altogether. Surgery to remove endometriosis or the uterus is another treatment for endometriosis.

Endometrial hyperplasia

Hyperplasia is an abnormal thickening of the uterine lining. It's caused by an imbalance of estrogen to progesterone. Having too much estrogen makes the cells of the endometrium grow more than usual. A lack of progesterone prevents the endometrium from shedding. 

Endometrial hyperplasia usually happens during or after menopause, when periods become irregular and progesterone levels drop. Symptoms include:

  • Heavier or longer than normal menstrual periods
  • Bleeding between periods or after menopause
  • Anemia – low red blood cells – from heavy bleeding

Your doctor can prescribe progestin in a pill, injection, cream, or intrauterine device (IUD) to balance out your hormone levels and relieve symptoms. Though endometrial hyperplasia isn't cancer, without treatment it can sometimes turn into endometrial cancer.

Endometrial cancer

Also called uterine cancer, this is the most common cancer of the female reproductive tract. Endometrial cancer starts when cells in the endometrium change and multiply uncontrollably. This type of cancer most often happens after menopause. Nearly 80% of people diagnosed with endometrial cancer are age 55 or older.

Symptoms include:

  • Heavier  than usual bleeding during periods
  • Bleeding between periods or after menopause
  • Pain in the pelvis

The endometrium is the lining of the uterus. Each month, it builds up to prepare for a fertilized embryo to implant and then sheds during a menstrual period if there is no pregnancy. Several conditions affect the endometrium, including endometriosis, endometrial hyperplasia, and endometrial cancer. Any abnormal menstrual bleeding or pelvic pain could signal a problem and is worth talking to your doctor about.

What happens if the endometrium is thick? The endometrium normally thickens during the menstrual cycle to prepare for a fertilized egg to implant. A thicker than normal endometrium can sometimes be a sign of endometrial hyperplasia or endometrial cancer.

What is the endometrium's normal size? The size ranges from 1 to 4 mm at the beginning of the menstrual cycle to about 14 mm at the end of the cycle. After menopause, the thickness is usually less than 5 mm.

What is an abnormal endometrium measurement? After menopause, the endometrium normally measures less than 5 mm. A thickness of more than 5 mm could signal a problem.