The endometrium is the mucosal lining of the uterus. This lining is one of the most dynamic tissues in the human body, undergoing a continuous cycle of growth, differentiation, and shedding. Hormones produced by the ovaries, specifically estrogen and progesterone, regulate the thickness of the endometrium, which is measured in millimeters (mm) using a transvaginal ultrasound. The measurement of this tissue is a common diagnostic tool used to investigate symptoms like abnormal uterine bleeding.
The Endometrial Lining and the Menstrual Cycle
The thickness of the endometrium fluctuates through three distinct phases during the menstrual cycle. In the menstrual phase, the lining is shed due to a drop in hormone levels, resulting in the thinnest measurement, typically ranging from 2 to 4 mm. The subsequent proliferative phase, driven by rising estrogen levels, sees the lining rapidly thicken as new tissue is built up. During this period, the endometrial thickness can increase from 5–7 mm in the early stage to as much as 11 mm by the late proliferative stage just before ovulation.
Following ovulation, the secretory phase is driven by high levels of progesterone, which causes the endometrium to mature and become ready for embryo implantation. This is when the lining reaches its maximum thickness, generally falling within a range of 7 to 16 mm. If pregnancy does not occur, hormone levels decline, and the cycle returns to the menstrual phase, beginning the process of shedding the thickened lining. The expected normal thickness is therefore dependent on the specific day of the cycle the measurement is taken.
Contextualizing a 14 mm Measurement
The interpretation of a 14 mm endometrial thickness depends on a person’s current hormonal status and whether they are experiencing any abnormal bleeding. For a pre-menopausal woman, 14 mm is generally considered a normal finding if obtained during the secretory phase. This thickness falls well within the expected 7–16 mm range, when the lining is at its most developed state. Therefore, 14 mm is not typically a cause for alarm in the context of a regular cycle.
Even in pre-menopausal women, a 14 mm thickness combined with abnormal uterine bleeding (AUB) may prompt further investigation. While the measurement itself is normal for the phase, persistent or erratic bleeding suggests the tissue may not be shedding properly or that other pathology is present. If the measurement were taken in the early proliferative phase, a thickness of 14 mm would be considered abnormally high and would require diagnostic follow-up.
The situation changes for post-menopausal women not taking hormone replacement therapy (HRT). In this group, the endometrium should be thin due to low hormone levels, with a threshold of 5 mm or less used to exclude most serious pathology. A 14 mm thickness in a post-menopausal woman is associated with an increased risk for endometrial hyperplasia or cancer. This finding requires tissue sampling.
For women taking HRT, the specific regimen influences the expected thickness. Women on continuous combined HRT, which delivers both estrogen and progestin daily, should maintain a thin lining of less than 5 mm. If 14 mm is found on this regimen, it is concerning and suggests an inadequate progestin effect or other underlying issue. Conversely, women on sequential HRT experience a planned buildup and shedding, and a transient thickness of 14 mm may be expected during the estrogen-dominant phase.
Diagnostic Follow-Up for Abnormal Thickness
When an endometrial thickness, such as 14 mm, is found to be outside the expected range for the patient’s hormonal status, a diagnostic pathway is initiated. The initial assessment is typically performed using transvaginal ultrasound (TVS) to measure the endometrial thickness. If the TVS reveals a thick lining, especially in a post-menopausal woman, or if a focal abnormality like a polyp is suspected, further visualization may be needed.
Saline Infusion Sonohysterography (SIS) is often the next step, involving the instillation of sterile saline into the uterine cavity during an ultrasound. This fluid distends the cavity, allowing for a clearer differentiation between diffuse thickening and focal lesions like polyps or fibroids. SIS helps determine the exact location and nature of the thickening before an invasive procedure is planned.
The primary diagnostic step for any abnormal endometrial thickening is tissue sampling, usually performed via an endometrial biopsy. This office-based procedure removes a small sample of the lining. The sample is then analyzed under a microscope to determine the cause of the thickening, such as benign hyperplasia, polyps, or malignant changes.
If the biopsy is inconclusive, or if a focal lesion is identified, a hysteroscopy may be performed. Hysteroscopy involves inserting a thin, lighted scope through the cervix to allow for direct visual inspection of the uterine cavity. This procedure allows for targeted removal of specific lesions or a more thorough tissue sampling.