Your uterine lining is called the endometrium. When you have an ultrasound or MRI, your endometrium will show up as a dark line on the screen. This line is sometimes referred to as the “endometrial stripe.” This term doesn’t refer to a health condition or diagnosis, but to a normal part of your body’s tissue.

Endometrial cells can appear in other parts of your body as a symptom of endometriosis, but an “endometrial stripe” specifically refers to endometrial tissue in your uterus.

This tissue will naturally change as you age and move through different reproductive stages. Read on to learn more about these changes, symptoms to watch for, and when to see your doctor.

If you’re of reproductive age, the overall appearance of your endometrial stripe will depend on where you are in your menstrual cycle.

Menstrual or early proliferative phase

The days during your period and immediately after it are called the menstrual, or early proliferative, phase. During this time, the endometrial stripe will look very thin, like a straight line.

Late proliferative phase

Your endometrial tissue will begin to thicken later in your cycle. During the late proliferative phase, the stripe may appear to be layered, with a darker line that runs through the middle. This phase ends once you’ve ovulated.

Secretory phase

The part of your cycle between when you ovulate and when your period starts is called the secretory phase. During this time, your endometrium is at its thickest. The stripe accumulates fluid around it and, on an ultrasound, will appear to be of equal density and color throughout.

The normal range of thickness varies according to what stage of life you’re in.

Pediatric

Before puberty, the endometrial stripe looks like a thin line all month long. In some cases, it may not yet be detectable by an ultrasound.

Premenopausal

For women of reproductive age, the endometrial stripe thickens and thins according to their menstrual cycle. The stripe can be anywhere from slightly less than 1 millimeter (mm) to slightly more than 16 mm in size. It all depends on what phase of menstruation you’re experiencing when the measurement is taken.

Average measurements are as follows:

  • During your period: 2 to 4 mm
  • Early proliferative phase: 5 to 7 mm
  • Late proliferative phase: Up to 11 mm
  • Secretory phase: Up to 16 mm

Pregnancy

When pregnancy occurs, a fertilized egg will implant into the endometrium while it is at its thickest. Imaging tests done during early pregnancy may show an endometrial stripe of 2 mm or more.

In a routine pregnancy, the endometrial stripe will become home to the growing fetus. The stripe will eventually be obscured by a gestational sac and placenta.

Postpartum

The endometrial stripe is thicker than usual after childbirth. That’s because blood clots and old tissue can linger after delivery.

These remnants are seen after 24 percent of pregnancies. They’re particularly common after cesarean delivery.

The endometrial stripe should return to its regular cycle of thinning and thickening when your period cycle resumes.

Postmenopausal

The thickness of the endometrium stabilizes after you reach menopause.

If you’re close to reaching menopause but still have occasional vaginal bleeding, the average stripe is less than 5 mm thick.

If you no longer experience any vaginal bleeding, the average stripe is between 8 and 11 mm. An endometrial stripe of 11 mm or more is considered to be thick at the postmenopausal stage.

Unless you’re experiencing unusual symptoms, thick endometrial tissue generally isn’t a cause for concern. In some cases, a thick endometrial stripe may be a sign of:

Polyps

Endometrial polyps are tissue abnormalities found in the uterus. These polyps make the endometrium appear thicker in a sonogram. In most cases, polyps are benign. In a small percentage of cases, endometrial polyps can become malignant.

Fibroids

Uterine fibroids can attach to the endometrium and make it look thicker. Fibroids are extremely common, with up to 80 percent of women developing them at some point before they turn 50.

Tamoxifen use

Tamoxifen (Nolvadex) is a drug used to treat breast cancer. Common side effects include early menopause and changes in the way your endometrium thickens and thins.

Endometrial hyperplasia

Endometrial hyperplasia occurs when your endometrial glands cause the tissue to grow more quickly. This condition is more common in women who have reached menopause. In some cases, endometrial hyperplasia can become malignant.

Endometrial cancer

According to the American Cancer Society, almost all uterine cancers start in the endometrial cells. Having an abnormally thick endometrium could be an early sign of cancer. Other symptoms include heavy, frequent, or otherwise irregular bleeding, irregular discharge after menopause, and lower abdominal or pelvic pain.

Unless you’re experiencing unusual symptoms, thin endometrial tissue generally isn’t a cause for concern. In some cases, a thin endometrial stripe may be a sign of:

Menopause

Your endometrium will stop its monthly thinning and thickening during and after menopause.

Atrophy

Low estrogen levels can lead to a condition called endometrial atrophy. Most often, this is connected to the onset of menopause. Hormone imbalances, eating disorders, and auto-immune conditions can also lead to atrophy in younger women. When your body has a low estrogen level, your endometrial tissue may not become thick enough for an egg to implant.

When endometrial cells grow at an abnormal rate, other symptoms can result.

If you have a thicker than normal endometrial stripe, these symptoms may include:

  • breakthrough bleeding between periods
  • extremely painful periods
  • difficulty getting pregnant
  • menstrual cycles that are shorter than 24 days or longer than 38 days
  • heavy bleeding during your period

If your endometrium is thinner than normal, you may have some of the same symptoms associated with thicker tissue. You may also experience:

If you’re experiencing any of these symptoms, make an appointment with your doctor. They may recommend an ultrasound or other diagnostic test to determine the cause.

Don’t hesitate to ask your doctor questions about your reproductive health. Your doctor can review your medical history and discuss what’s normal for you.

If you’re experiencing abnormal symptoms, make sure to see your gynecologist — you shouldn’t wait until your yearly exam. Doing so could delay any necessary treatment.