The endometrium is the highly specialized tissue lining the inside of the uterus, and its primary function is to prepare for and support a potential pregnancy. This lining undergoes dramatic monthly changes in thickness, driven by the fluctuating levels of estrogen and progesterone hormones. Measuring the endometrial thickness, typically done with a transvaginal ultrasound, is a routine procedure in gynecology because its size and appearance can provide significant insight into a person’s hormonal status and reproductive health. The normal range for this measurement is highly variable, depending entirely on whether the individual is cycling, postmenopausal, or undergoing specific fertility treatments.
Normal Thickness During the Menstrual Cycle
The thickness of the endometrium changes predictably across the phases of the menstrual cycle, reflecting the preparation for potential implantation. At the beginning of the cycle, during menstruation, the lining is at its thinnest, typically measuring between 2 and 4 millimeters (mm) as the functional layer is shed. This phase is known as the menstrual or early proliferative phase.
As the body enters the mid-proliferative phase, under the influence of rising estrogen levels, the endometrium begins to regenerate and thicken. During this time, which is approximately days 6 through 14, the thickness usually increases to a range of 5 to 10 mm. On ultrasound, the lining often develops a distinct “trilaminar” or “three-line” appearance.
Following ovulation, the body shifts into the secretory phase, where progesterone dominates and prepares the lining to be receptive to an embryo. This is the thickest phase of the cycle, with measurements commonly ranging from 7 to 16 mm, and sometimes reaching up to 18 mm. The trilaminar pattern typically disappears as the tissue becomes more uniform and uniformly echogenic. An endometrial thickness less than 14 mm is generally considered within the normal range for any point during the reproductive cycle.
Postmenopausal Endometrial Thickness
The measurement of endometrial thickness takes on a different clinical significance after menopause, when hormonal stimulation has largely ceased. In postmenopausal individuals who are not taking Hormone Replacement Therapy (HRT), the lining should be relatively thin, often measuring 5 mm or less. A measurement exceeding this 5 mm threshold in a patient experiencing postmenopausal bleeding is usually considered an indication for further investigation, such as an endometrial biopsy, to rule out pathology.
The risk of endometrial cancer is extremely low when the measurement is 5 mm or less in a symptomatic patient, but the risk increases to around 7% when the thickness exceeds this level. For asymptomatic postmenopausal women, the threshold for concern is higher, often ranging between 8 mm and 11 mm before investigation is recommended. The interpretation of the measurement is also affected by the use of hormonal medications.
Women taking HRT, particularly combined regimens, can have a slightly thicker lining, often allowed to be up to 8 to 11 mm before triggering a biopsy. This increased threshold reflects that the hormones used in HRT stimulate some endometrial growth, making the strict 5 mm limit less applicable.
Clinical Significance: Endometrial Thickness and Implantation
In assisted reproductive technology (ART), such as In Vitro Fertilization (IVF), the endometrium’s thickness and appearance are monitored to optimize successful embryo implantation. The consensus among fertility specialists is that the ideal endometrial thickness for embryo transfer typically falls within a range of 8 to 14 mm. Studies have shown that clinical pregnancy and live birth rates are significantly higher when the lining is at least 7 mm thick at the time of transfer.
A lining measuring less than 7 mm is often classified as a “thin endometrium,” which is associated with decreased success rates for implantation. The quality of the lining, including the presence of the trilaminar appearance, is considered alongside the numerical measurement. Although successful pregnancies have been reported with thinner linings, fertility protocols aim for a thickness that maximizes the likelihood of successful attachment and support.