The endometrium is the tissue that lines the inside of the uterus. When viewed on a transvaginal ultrasound, this lining appears as a bright, central stripe, commonly referred to as the “endometrial stripe” thickness. This routine, non-invasive measurement helps healthcare providers assess a person’s hormonal status and check for potential uterine issues. The thickness is highly dynamic, fluctuating dramatically based on the patient’s age and hormonal environment. Therefore, a single measurement only gains meaning when considered within the context of the individual’s menstrual cycle or menopausal status.
The Endometrium’s Hormonal Cycle
The thickness of the endometrium is not static in pre-menopausal individuals, changing constantly in response to circulating hormones. The cycle begins with the menstrual phase, during which the old lining is shed, leaving the endometrial stripe at its thinnest. Following this shedding, the proliferative phase begins, driven primarily by the hormone estrogen.
Estrogen stimulates the growth and thickening of the uterine lining, preparing it to receive a fertilized egg. This period sees the stripe rapidly increase in thickness and develop a characteristic “trilaminar” or three-layered appearance on ultrasound. After ovulation, the secretory phase takes over, with progesterone becoming the dominant hormone. Progesterone causes the lining to become more vascular and spongy, maximizing its thickness for optimal embryo implantation. If pregnancy does not occur, hormone levels drop, leading to the breakdown and shedding of the lining, which restarts the cycle.
Standard Measurements of Thickness
The normal range for endometrial stripe thickness is highly specific to the patient’s hormonal status, making context essential for interpretation.
Pre-menopausal Women
The thickness varies significantly throughout the month for women who are still menstruating. It is thinnest during the menstrual and early proliferative phases, typically measuring between 2 and 4 millimeters (mm). As the proliferative phase continues, the lining thickens, reaching up to 11 mm just before ovulation. The maximum thickness is seen during the secretory phase, where the stripe can range from 7 mm to 16 mm.
Post-menopausal Women Not on HRT
For post-menopausal women not using Hormone Replacement Therapy (HRT), the typical thickness is much lower due to the drop in estrogen. In this group, an endometrial stripe measurement of 4 to 5 mm or less is considered the upper limit of normal. A measurement above this threshold may prompt further investigation, particularly if the patient is experiencing post-menopausal bleeding.
Post-menopausal Women on HRT
The use of HRT introduces hormonal stimulation, causing the endometrial stripe to be thicker than in non-users. For women on continuous combined HRT (estrogen and progesterone), the target thickness is often set at 5 mm or less. Women on sequential or cyclical HRT, which mimics a menstrual cycle, may have an acceptable thickness of up to 8 mm. This measurement should ideally be taken when the lining is expected to be thinnest, shortly after the progestin-induced bleed.
Clinical Implications of Increased Thickness
An endometrial stripe significantly thicker than expected raises concern for abnormal cell growth. This is particularly true for post-menopausal women, where a measurement over 5 mm, especially with bleeding, warrants immediate investigation. The primary concern is Endometrial Hyperplasia, a condition where the lining grows excessively due to estrogen overstimulation without adequate progesterone counter-effect.
Hyperplasia ranges from simple, non-cancerous overgrowth to atypical hyperplasia, which is considered a precursor to Endometrial Cancer. Other non-malignant causes of increased thickness include Endometrial Polyps, which are localized growths protruding into the uterine cavity. When a thick stripe is found, especially in a post-menopausal patient, the diagnostic process typically moves to a hysteroscopy or an endometrial biopsy. These procedures obtain tissue samples to determine the specific cause of the thickening and ensure prompt treatment.
Clinical Implications of Decreased Thickness
A stripe that is too thin, often defined as less than 3 to 4 mm in a reproductive-age person, presents clinical challenges. In post-menopausal women, a very thin lining is usually the result of Endometrial Atrophy, a benign and expected finding due to low estrogen levels. Conversely, in individuals trying to conceive, a thin lining can impact fertility outcomes.
For those undergoing Assisted Reproductive Technology (ART), such as In Vitro Fertilization (IVF), a thin endometrium is associated with lower clinical pregnancy and live birth rates because it impedes successful embryo implantation. While the absolute threshold varies, many studies suggest that an endometrial stripe below 7 mm on the day of embryo transfer is suboptimal. Certain medications, including long-term use of combined oral contraceptives or GnRH agonists, can suppress the lining and cause it to thin. A thin lining signals a potential hormonal imbalance or a challenge to reproductive success, though it is less likely to indicate cancer.