Endometrial Thickness

Endometrial thickness is the depth of the endometrium, the uterine lining, measured by ultrasound. The scan is performed through the vagina (transvaginal ultrasound). Clinicians assess the lining at specific points in the cycle. Before ovulation, a healthy lining often shows three clear layers on the scan. This pattern shows that estrogen is acting on the lining. A lining that remains thin before ovulation is linked to a lower chance that an embryo will implant. Exact cut-off values are not settled. They differ by the group studied and by the type of cycle.

A thin lining has underlying causes. They include past surgery inside the uterus, such as dilation and curettage. Scar tissue inside the uterus (Asherman syndrome) is another cause. Others include low estrogen, poor blood flow to the uterus, and chronic infection. When a clinician finds a thin lining, the next question is why. The number alone does not answer that question.

Thickness is one part of the clinical picture. It belongs next to ovulation timing, hormone testing, and an examination of the uterine cavity. A normal number does not rule out a lining that cannot support a pregnancy. A thin lining can also improve once the cause is treated. The window of implantation depends on more than thickness. It opens after ovulation, when progesterone changes the lining. The window is counted from ovulation, not from a set calendar day. In one study of natural cycles, implantation occurred 6 to 12 days after ovulation. Most implantation occurred on day 8, 9, or 10.¹