The endometrium is the mucous membrane lining of the uterus. It is sometimes referred to as the endometrial stripe because it shows up as a dark stripe on transvaginal ultrasound. The size of the stripe, or the thickness of the endometrium, can be an indicator of endometrial abnormality, though this must be considered along with other signs or symptoms a woman may be experiencing.
The endometrium plays an important role in a woman's fertility cycle. During childbearing years, it averages anywhere from 1 to 15 millimeters in thickness, increasing to 14 to 16 millimeters after ovulation. At the beginning of the cycle, the endometrium begins to thicken in preparation for an ovum, or egg, to become fertilized and implant itself into the uterine lining. If fertilization occurs, the endometrium thickens even more and becomes the decidua, part of the placenta. If fertilization does not occur, the endometrium is shed in the menstrual cycle.
Abnormal Uterine Bleeding
The size of the endometrial stripe on a transvaginal ultrasound can help define the possible causes of abnormal uterine bleeding. Abnormal bleeding in the presence of a thick endometrial stripe may indicate a different condition other than abnormal bleeding in the presence of a thin endometrial stripe. Each may require different testing and treatment. Any post-menopausal bleeding in the absence of initial hormone replacement therapy is abnormal and should prompt a visit to a doctor.
A thickened endometrial stripe may indicate hyperplasia, an overgrowth of the lining of the uterus beyond the normal thickness. This is usually due to hormonal imbalance between estrogen and progesterone--that is, too much estrogen and not enough progesterone. There are four types of endometrial hyperplasia, categorized by how likely each type is to lead to endometrial cancer. These are simple (1 percent chance), complex (3 to 5 percent chance), simple with atypia (8 to 10 percent chance) and complex with atypia (25 to 30 percent chance).
Though a thickened endometrial stripe is seen with endometrial cancer, it is not by itself indicative of cancer. A stripe greater than 5 millimeters is suggestive of hyperplasia, and it is the evaluation of the hyperplasia that will indicate whether cancer is a possibility. Atypical hyperplasia is an indication for an endometrial biopsy. Post-menopausal women are more at risk for endometrial cancer than pre-menopausal women. Of women who do have endometrial cancer, 75 percent are post-menopausal.
On a transvaginal ultrasound, what may appear as a thickened endometrial stripe may not be the endometrium at all. Endometrial polyps or fibroids may distort the endometrial stripe, making it look thicker. In such cases, further studies may be needed. Women treated with tamoxifen (for breast cancer) may also have a thickened endometrial stripe and should be monitored, whether symptomatic or asymptomatic. Also, a thin endometrial stripe (due to menopause and atrophy of the endometrium) does not preclude the possibility of a non-estrogen-dependent cancer.