Get to Know Your Endometrium

How the Endometrium Plays an Important Role in Your Reproductive Health

The human uterus, showing the boundary between the endometrial glands and the smooth muscle. (Magnification x100)
Garry DeLong / Getty Images

The endometrium is the innermost of the three layers of tissue that line the uterus (the expandable organ in which a fetus is nourished and develops before birth). You could think of it as the wallpaper of the womb, but the functions it performs are well beyond decorative. Sometimes called the endometrial lining, the endometrium plays key roles in the menstrual cycle and in pregnancy. Here's a look at why the endometrial lining is so important to these phases of the reproductive cycle.

The Endometrium and Pregnancy

The endometrium is made up mostly of mucosal tissue and has two layers. One of these stays relatively constant and is where the endometrium attaches to the middle layer of the endometrium (smooth muscle tissue called the myometrium.)

The other layer, however, is dynamic—it changes in response to the monthly flux of hormones that guide the menstrual cycle. For this reason, it's called the functional layer. Because it's the part of the endometrium where an egg will implant if it's fertilized, during each cycle before ovulation (the release of an egg from a Fallopian tube), the functional layer of the endometrium goes through all sorts of changes just in case fertilization happens. Structures called uterine glands get longer and blood vessels proliferate.

To trigger ovulation, the levels of the hormone progesterone increases. This causes the thickened endometrial lining to mature.

The glands and arteries come together to get ready to allow an egg to implant (if it's fertilized) and to prepare for the development of the placenta. (The organ that develops during a pregnancy to nourish a developing fetus.) 

The Endometrium and Your Period

What happens if an egg is released and doesn't get fertilized?

In that case, all that preparation for pregnancy was for nothing, and the blood that helped to fatten up the functional layer of the endometrium is no longer needed and must be shed.

This shedding is your period. Menstrual flow is made up of the cells that are shed from the functional layer mixed with blood from the little blood vessels that surrounded the glands. As the next cycle begins, the functional layer regrows under the influence of estrogen

How Hormones Affect The Endometrium

Keeping in mind that the endometrium changes in response to hormone levels just before ovulation, it makes sense that in girls who haven't started menstruation and in menopausal women who've stopped having periods, the endometrium is relatively thin and doesn’t change. Both of these groups will have no monthly flow.

However, an excess of hormones can stimulate the endometrium to become ampler than is typical. For instance, the excess fat cells in women who are dealing with obesity produce excess estrogen as well. This can lead to an extra build up of the endometrium and, ultimately, heavier periods. When chronic, this is a condition called endometrial hyperplasia.

On the other hand, hormonal birth control methods can have the opposite effect.

Women 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.


Maybin, JA. Menstrual Physiology: Implications For Endometrial Pathology and Beyond. Human Reprod Update. 2015;21(6):748-761

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