The endometrial stripe, or endometrial lining, is the tissue layer that lines the inside of the uterus. Gynecologists measure this tissue in millimeters using a transvaginal ultrasound to assess uterine health and investigate abnormal uterine bleeding. The lining’s thickness directly reflects hormonal activity and can signal various conditions, including benign growths or malignancy. The specific thickness considered normal depends entirely on a person’s age and hormonal status.
Understanding Cyclical Changes in Premenopausal Women
The thickness of the endometrial stripe in premenopausal women changes throughout the menstrual cycle in response to estrogen and progesterone. The cycle is divided into phases, each with a distinct expected range of thickness.
During menstruation and the early proliferative phase, the lining is thinnest, typically measuring between 2 to 4 millimeters (mm) as the functional layer is shed. Rising estrogen levels then stimulate the regrowth of the endometrium during the late proliferative phase. This growth sees the lining thicken to around 5 to 10 mm, often developing a characteristic trilaminar, or “three-layered,” appearance on ultrasound.
After ovulation, the secretory phase begins, driven by progesterone, which prepares the lining for a potential pregnancy. In this phase, the endometrium reaches its maximum thickness, ranging from 7 to 16 mm. If pregnancy does not occur, hormonal support drops, and the cycle repeats with the shedding of the thickened lining.
The Critical Diagnostic Thresholds for Postmenopausal Women
For women who have not had a menstrual period for 12 months, the hormonal environment changes, and the expected endometrial thickness becomes much thinner. The absence of cyclical hormones means the lining is no longer stimulated to grow and shed monthly. A thin endometrial stripe in this population is a reassuring sign, often indicating endometrial atrophy due to low estrogen levels.
The primary clinical concern is abnormal uterine bleeding, and thickness measurement guides the need for further investigation. For a postmenopausal woman not taking hormone replacement therapy (HRT), an endometrial stripe measuring 4 to 5 mm or less is considered the upper limit of normal. If the measurement is below this threshold, the risk of endometrial cancer is extremely low, and no further invasive procedure is required to investigate bleeding.
If a woman experiences postmenopausal bleeding and the stripe measures greater than 5 mm, there is an increased risk of endometrial hyperplasia or cancer. This measurement prompts a physician to recommend a biopsy for tissue diagnosis. For asymptomatic postmenopausal women (no vaginal bleeding), the threshold for concern is higher, with evaluation warranted if the thickness exceeds 11 mm.
How Medications and Hormonal Therapies Alter Expected Thickness
Certain medical treatments and hormonal therapies can intentionally or unintentionally override the natural thickness parameters, requiring different monitoring standards. Hormone Replacement Therapy (HRT) is a common example, as it reintroduces hormones that directly affect the endometrium. Women on continuous combined HRT, which includes both estrogen and progesterone, generally have a thin, atrophic lining, with a thickness of 5 mm or less being the expected range.
However, women on sequential or unopposed estrogen HRT will experience endometrial growth due to the estrogen, making the acceptable thickness range greater than the standard postmenopausal threshold. The acceptable upper limit for these patients can be up to 8 mm, and any measurement exceeding this may warrant further investigation.
The breast cancer drug Tamoxifen presents a unique situation because it acts as an anti-estrogen in breast tissue but can have estrogen-like effects on the endometrium. This medication commonly causes the endometrial lining to thicken significantly, often resulting in measurements between 9 to 13 mm, even in the absence of any disease. Because Tamoxifen can lead to benign polyps or hyperplasia, a different monitoring strategy is used, and abnormal bleeding or a sudden increase in thickness should still be promptly evaluated.
Fertility treatments also intentionally manipulate the endometrial thickness to optimize the chances of implantation. Protocols using high doses of estrogen and progesterone are designed to thicken the lining to an optimal range, often cited as 7 to 14 mm, to create a receptive environment for an embryo. In this context, a thick lining is a desired outcome, while a lining thinner than 7 mm may be associated with reduced implantation rates.
When Endometrial Thickness Signals a Need for Further Evaluation
A measurement that falls outside the expected range for a person’s age and hormonal status is considered a signal for further clinical evaluation, not a diagnosis in itself. An excessively thick stripe can be caused by a variety of conditions, including benign endometrial polyps, fibroids, or a hormonal imbalance resulting from unopposed estrogen stimulation. In the most concerning cases, a thickened endometrium may be a sign of precancerous changes or endometrial cancer, particularly in the postmenopausal population.
Conversely, an unusually thin endometrial stripe may also require evaluation, especially in premenopausal women experiencing infertility or abnormal bleeding. A thin lining, sometimes defined as 7 mm or less, can be associated with conditions like endometrial atrophy, chronic inflammation, or scarring within the uterine cavity, such as Asherman’s syndrome. In the context of fertility, a thin lining can hinder the successful implantation of an embryo.
If the ultrasound measurement is a cause for concern, physicians typically recommend follow-up procedures to obtain a definitive diagnosis. The most common next step is an office-based endometrial biopsy, which removes a small tissue sample for laboratory analysis. If the initial biopsy is inconclusive, or if a focal abnormality like a polyp is suspected, a hysteroscopy may be performed, allowing the physician to visualize the uterine cavity directly and perform a targeted biopsy or removal of the tissue.