The endometrium undergoes cyclical changes throughout the menstrual cycle. During menstruation, the endometrium sheds completely and becomes relatively thin. After menstruation, as estrogen levels rise, the endometrium begins to proliferate. By the time of ovulation and shortly after, the endometrial thickness can reach up to 13 millimeters, which is considered normal.

However, if the menstrual cycle is regular and the endometrium measures 13 millimeters immediately after menstruation, it may indicate endometrial abnormalities. Conditions such as endometrial polyps or submucosal fibroids could be present, and clinical symptoms should be considered for diagnosis. If accompanied by prolonged menstrual periods or heavy bleeding, a hysteroscopy should be performed 3 to 7 days after the menstrual cycle ends. It is advisable to obtain an endometrial sample for pathological examination to confirm the diagnosis.

If the endometrial thickness is 13 millimeters just before menstruation or during early pregnancy, it is considered normal and does not require additional intervention.