Proliferative endometrium

What does proliferative endometrium mean?

Proliferative endometrium is a very common non-cancerous change that develops in the tissue lining the inside of the uterus. It is a normal finding in women of reproductive age.

During the menstrual cycle, the endometrium grows under the influence of two major hormones – estrogen and progesterone. In the first part of the menstrual cycle, between menses and ovulation (ovulation is when a mature egg is released from the ovary, pushed down the fallopian tube, and is made available to be fertilized), the endometrium grows under the influence of estrogen. Pathologists call this phase the proliferative endometrium.

These changes occur during the fertile age, which for most women starts at early teens, and lasts until the age of 45-55. Menstrual flow might occur every 21 to 35 days and usually lasts two to seven days.

The uterus and endometrium

The uterus is a hollow muscular organ located in the female pelvis between the urinary bladder and the rectum (the lower end of the large bowel). The upper part of the uterus, the fundus, is attached to the fallopian tubes while the lower part is connected to the vagina through the uterine cervix.  Functions of the uterus include nurturing the baby, and holding it until the baby is mature enough for birth.

The walls of the uterus are made up of three layers:

1. Endometrium – The endometrium forms the inner lining of the uterus. The endometrium is composed of endometrial glands lined by one layer of columnar epithelium and surrounded by endometrial stroma.
2. Myometrium – The myometrium is the middle layer and is made up of smooth muscle which allows the uterus to change size and contract.
3. Perimetrium – The perimetrium is a thin layer of tissue that surrounds the outside of the uterus.

Changes during the menstrual cycle

The endometrium is hormone-responsive which means it changes in response to hormones released during the menstrual cycle. Following every menstrual period (menses) the endometrium grows to a thick, blood vessel-rich, glandular tissue layer, providing an optimal environment for a fertilized egg.

If the fertilization does not occur, the endometrium breaks down, leaving only the bottom layer (basal layer) and many open blood vessels. This leads to a temporary bleed and discharge of blood and endometrial tissue through the vagina (menstruation, menstrual period, menstrual flow). Once the menstruation is over, the endometrium starts growing again, and the cycle repeats.

How do pathologists make this diagnosis?

The diagnosis of proliferative endometrium is usually made after a small sample of tissue is removed from the endometrium during a procedure called an endometrial biopsy or uterine curetting.

Common reasons for these procedures include:

• Abnormal (dysfunctional) uterine bleeding.
• Postmenopausal bleeding.
• Screening for endocervical or endometrial cancer.
• Endometrial dating.
• Follow up of previously diagnosed endometrial hyperplasia.
• Endometrial or endocervical polyps.
• Infertility.

Your pathology report may also say negative for hyperplasia and malignancy, which means there was no evidence of endometrial cancer or endometrial hyperplasia in the tissue samples examined. Based on this diagnosis, your doctor will suggest further testing and treatment.

by Adnan Karavelic MD FRCPC (updated September 21, 2021)