The Endometrium and Its Role in Reproductive Health

The endometrium is the tissue that lines the uterus, or womb—the pear-shaped organ that houses a fetus. This mucous membrane thickens in anticipation of a possible pregnancy. If a fertilized egg is implanted, the lining stays in place. If not, the endometrium thins and sheds as a menstrual period.

Abnormalities of the endometrium, also called the endometrial lining, can result in concerns such as endometriosis (tissue grows outside the uterus), hyperplasia (lining thickens), and cancer.

This article discusses the endometrium, its purpose, and conditions that can affect the this tissue. It also explains how the endometrium's size and thickness can affect your fertility and health.

Anatomy of the Endometrium

The endometrium is made up mostly of mucosal tissue. It has two layers.

The first layer, the stratum basalis, attaches to the layer of smooth muscle tissue of the uterus called the myometrium. This layer serves as an anchor for the endometrium within the uterus and stays relatively unchanged.

The second layer is dynamic, however. It changes in response to the monthly flux of hormones that guide the menstrual cycle. For this reason, it's called the stratum functionalis, or functional layer. It's the part of the endometrium where a fertilized egg (or blastocyst) will implant if conception takes place.

Menstruation and Pregnancy

Just before ovulation (the release of an egg from a fallopian tube), the functional layer of the endometrium goes through specific changes. Structures called uterine glands become longer, and tiny blood vessels proliferate—a process called vascularization.

As a result, the endometrial lining becomes thicker and enriched with blood so that it's ready to receive a fertilized egg and also support a placenta—the organ that develops during pregnancy to supply a fetus with oxygen, blood, and nutrients.

If conception doesn't take place after ovulation, the build-up of blood vessels and tissues becomes unnecessary and is shed. This is your period.

Keep in mind that those too young to have gotten their periods yet and adults who've gone through menopause will not experience these changes. Their endometrial linings will stay relatively thin and stable.

Hormonal birth control methods can have a similar effect. People who use progesterone-only contraception such as the Mirena intrauterine device or the contraceptive implant Nexplanon, both of which ultimately suppress the build-up of the functional layer of the endometrium, tend to have lighter periods.

Conditions That Affect the Endometrium

Most of the time, the ebb and flow of the endometrial lining follow a fairly predictable rhythm—as you well know if you're a person who menstruates. However, this can be altered by abnormalities of the endometrial lining. Here are the most common ones women may experience.

Endometriosis

Sometimes as it thickens, the endometrial lining wanders outside of the borders of the uterus and builds upon the ovaries, fallopian tubes, or tissue that lines the pelvis.

Even though it's outside of the uterus, this tissue will continue to grow and then break down as you menstruate. The problem is because it is displaced, the blood and tissue have nowhere to exit the body and become trapped.

Eventually, endometriosis can lead to cysts on the ovaries called endometriomas, as well as scar tissue and adhesions that cause structures in the pelvis to stick together.

The main symptom is severe pain—not only during menstruation but also during intercourse, bowel movements, or urination. Periods may be heavy, and you may feel extra tired, bloated, or nauseous.

About 40% of people who have endometriosis will have some degree of infertility resulting from any number of complications, such as scar tissue and adhesions in and around the fallopian tubes to low levels of progesterone that can affect the build-up of the uterine lining—a condition called luteal phase defect.

Endometrial Hyperplasia

In this condition, the endometrial lining becomes too thick. This most often happens due to a specific hormonal imbalance.

An excess of estrogen, which causes the thickening of the endometrium, in combination with an absence of progesterone, can result if ovulation doesn't take place. Under these conditions, the endometrial lining isn't shed, and cells within it continue to proliferate.

Endometrial hyperplasia can occur during perimenopause when ovulation becomes irregular or after menopause when the menstrual cycle stops altogether. It also can happen in people who take medications that act like estrogen (without progestin or progesterone) or who take high doses of estrogen after menopause for a long period of time.

Other risk factors include irregular menstrual periods, particularly in people who have polycystic ovary syndrome (PCOS), are infertile, or are obese: Excess fat cells produce excess estrogen as well. This can lead to an extra buildup of the endometrium and, ultimately, heavier periods.

The symptoms of endometrial hyperplasia include menstrual bleeding that's heavier or lasts longer than usual, shorter than normal periods, or any bleeding after menopause. If you experience any of these symptoms, see your gynecologist.

Endometrial hyperplasia can put you at risk of endometrial cancer, as the excess cells can become abnormal. The condition is usually treated with progestin.

Endometrial Cancer

Endometrial cancer is caused by the growth of abnormal cells. About 90% of people who are diagnosed with this condition have abnormal vaginal bleeding.

Other possible symptoms of endometrial cancer include non-bloody vaginal discharge, pelvic pain, feeling a mass in your pelvic area, or unexplained weight loss.

If your periods change dramatically (they become heavier or last longer, for example) or you have bleeding between periods or after you go through menopause, see your healthcare provider. There are less serious causes for these symptoms, but it's better to err on the side of caution.