The endometrium is the lining of the uterus. It is one of the few organs in the human body that changes in size every month throughout a person’s fertile years.
Each month, as part of the menstrual cycle, the body prepares the endometrium to host an embryo. Endometrial thickness increases and decreases during the process.
Two hormones, estrogen and progesterone, prompt these cycles of endometrial growth and its shedding through menstruation if a pregnancy does not develop.
In this article, we look at the normal range for endometrial thickness, causes of changes, and when to see a doctor.
The normal thickness of the endometrium changes throughout a person’s life, from childhood, through to sexual maturity, fertile years, and after menopause.
In imaging tests of young females who have not yet begun menstruating, the endometrium is present but smaller than it will be later in life.
According to the Radiological Society of North America (RSNA), the endometrium is at its thinnest during menstruation, when it usually measures between 2–4 millimeters (mm) in thickness.
The first half of the proliferative phase starts around day 6 to 14 of a person’s cycle, or the time between the end of one menstrual cycle, when bleeding stops, and before ovulation. At this phase, the endometrium begins to thicken and may measure between 5–7 mm.
As the cycle progresses and moves towards ovulation, the endometrium grows thicker, up to about 11 mm.
About 14 days into a person’s cycle, hormones trigger the release of an egg. During this secretory phase, endometrial thickness is at its greatest and can reach 16 mm.
Endometrial thickness is important in pregnancy. Healthcare experts link the best chances for a healthy, full-term pregnancy to an endometrium that is neither too thin nor too thick. This allows the embryo to implant successfully and receive the nutrition it needs. The endometrium gets thicker as the pregnancy progresses.
The RSNA also state that in healthy postmenopausal people, the endometrium typically measures about 5 mm or less.
How to measure
Ultrasound is the most common way to measure the thickness of the endometrium. It is the method that healthcare providers use first, especially if an individual has reported abnormal vaginal bleeding.
When ultrasound is not suitable, often due to the position of a person’s uterus or other health conditions, doctors use MRI.
Causes of a very thin or thick endometrial lining
The thickness of the endometrium changes during a person’s menstrual cycle, but other factors can prompt changes as well.
One of the more common causes of changes in endometrial thickness is pregnancy. Women who are having an ectopic pregnancy or who are less than 5 weeks pregnant may show signs of a thickening
Cancer of the endometrium or the ovaries is one of the most severe conditions that can lead to an increase in endometrial thickness. According to the American Cancer Society, endometrial cancer is the most common cancer affecting a person’s reproductive systems. Developing more often in white people than African American people, endometrial cancer is rare in females under 45. The average age at diagnosis is 60.
Other factors contributing to a greater thickness of the endometrium include:
- hormone replacement therapy (HRT)
- chronic high blood pressure
- endometrial polyps
- scar tissue
- endometrial hyperplasia
Endometrial hyperplasia is the medical term for a condition in which the endometrium becomes too thick. This is often related to excessive levels of estrogen or estrogen-like compounds, and not enough progesterone. The condition itself is not cancer, but it can lead to the development of cancer.
It is also possible for the endometrium to be too thin. Researchers define a thin endometrium as 7mm or less. Typically, experts associate low readings of endometrial thickness with age. However, they report that 5% of people under 40, and 25% of people over 40 had a thin endometrium.
Possible causes of a thin endometrium include inflammation, medical treatment, or the structure and nature of the endometrium itself.
The most common signs of excessive endometrial thickness include:
- extremely heavy or long-lasting bleeding during menstruation
- bleeding after menopause
- irregular menstrual cycles that last less than 3 weeks or longer than 38 days
- spotting between periods
Treatments for excessive endometrial thickness include progestin, a female hormone that prevents ovulation and hysterectomy.
Studies show that it is more difficult for a pregnancy to progress when readings for endometrial thickness are low. Treatments for a thin endometrium can include:
- human chorionic gonadotrophin, which is a hormone that the placenta produces after an embryo implants in the uterus wall
- medications and supplements used to improve blood flow
However, research shows that these treatments are not consistently effective.
When to see a doctor
People should see their doctors if they notice abnormal vaginal bleeding, which can include:
- spotting between period
- unusually heavy flows
- irregular, short blood flows
- vaginal bleeding or spotting after menopause
People experiencing pelvic pain of unknown origin should see a doctor for evaluation and to rule out endometrial cancer. Other symptoms to watch out for are bloating and a feeling of fullness without eating much.
Paying attention to endometrial thickness can help women who are trying to become pregnant understand the best way to optimize their chances of successful conception.
Changes in endometrial thickness are common throughout a person’s life. However, if someone notices abnormal bleeding, discharge, pelvic pain, or other changes in the way their body feels, they should consult a doctor to receive proper treatment.
Endometrial cancer is one of the most severe health problems that can occur if a person’s endometrium is too thick. However, endometrial cancer has a good survival rate if diagnosed early.
Written by Danielle Dresden
- Endometrial cancer. (2019).
- Endometrial cancer survival rates, by stage. (2019).
- Endometrial hyperplasia. (2012).
- Key statistics for endometrial cancer. (2019).
- Langer, J. E., et al. (2012). Imaging of the female pelvis through the life cycle.
- Mahajan, N., & Sharma, S. (2016). The endometrium in assisted reproductive technology: How thin is too thin?
- Monard, M., et al. (2018). Secretory phase of menstruation and implantation.
- Nalaboff, K. M., et al. (2001). Imaging the endometrium: Disease and normal variants.
- Normal periods. (n.d.).
- Progestin-only oral contraceptives. (2016).
- Sadro, C. T. (2016). Imaging the endometrium: A pictorial essay.
- What is endometrial cancer? (2019).