The endometrium is the inner lining of the uterus, a dynamic tissue that changes each month in preparation for potential pregnancy. It serves as the primary site for a fertilized egg to embed and develop, providing the necessary environment for early embryonic growth. This lining has two main layers: a stable basal layer and a functional layer that responds to hormonal signals. The functional layer thickens and sheds if pregnancy does not occur.
The Menstrual Cycle and Endometrial Changes
The endometrium’s thickness fluctuates throughout the menstrual cycle, driven by hormonal shifts. At the cycle’s beginning, during menstruation (days 1-5), the functional layer is shed, making the lining thinnest, usually 1-4 millimeters. Following menstruation, the proliferative phase (day 5-14) begins, during which the endometrium rebuilds and thickens.
As the cycle progresses, endometrial thickness increases from about 5-7 millimeters in early proliferative stage to 10-16 millimeters by the late proliferative or pre-ovulatory stage. After ovulation (around day 14), the cycle enters the secretory phase, lasting until menstruation if pregnancy does not occur. The endometrium continues to mature and thicken, reaching its maximum thickness.
The endometrium is thickest in the mid-to-late secretory phase, generally 7-18 millimeters, preparing for possible embryo implantation. This peak thickness usually occurs around days 20-24 of a typical 28-day cycle. If a fertilized egg does not implant, the thickened lining is shed, leading to the next menstrual period.
Hormonal Regulation of Endometrial Thickness
The thickening and maturation of the endometrium are orchestrated by two primary female hormones: estrogen and progesterone. Estrogen, produced by developing follicles, plays a central role in the initial growth phase. During the proliferative phase, rising estrogen levels stimulate endometrial cells to multiply, significantly increasing the lining’s thickness.
After ovulation, the ruptured follicle transforms into the corpus luteum, producing large amounts of progesterone. Progesterone then transforms the estrogen-primed endometrial lining into a secretory one. This makes the tissue more vascular and glandular, enhancing its readiness to receive and nourish a fertilized egg.
Progesterone also stabilizes the endometrial lining, preventing excessive growth that estrogen alone might cause. The coordinated rise and fall of these two hormones ensure the endometrium is prepared for implantation. Without this hormonal balance, the endometrial lining may not reach the optimal thickness or quality for a successful pregnancy.
The Role of Endometrial Thickness in Fertility and Health
An optimal endometrial thickness is important for successful embryo implantation and pregnancy. A lining generally between 7-14 millimeters in the secretory phase is often considered favorable for conception. For fertility treatments like in vitro fertilization (IVF), an endometrial thickness between 8-14 millimeters is typically sought for best outcomes.
When the endometrium is too thin (less than 7 millimeters), it can challenge implantation. This may be linked to low estrogen, uterine scarring (e.g., Asherman’s syndrome), or issues with uterine blood flow. A thin lining can lead to difficulties conceiving or an increased risk of early miscarriage.
Conversely, an excessively thick endometrium, known as endometrial hyperplasia, can impact reproductive health and indicate underlying issues. This often results from prolonged estrogen exposure without sufficient progesterone to balance its growth-promoting effects. Causes include hormonal imbalances, polycystic ovary syndrome (PCOS), obesity, or certain hormone therapies.
While often benign, endometrial hyperplasia can sometimes be a precursor to endometrial cancer, particularly if abnormal cell changes are present. Symptoms typically involve abnormal uterine bleeding, such as heavy or prolonged periods, or bleeding between cycles. Endometrial thickness is commonly assessed using transvaginal ultrasound for measurement and evaluation.