The normal size of the endometrium in mm typically ranges from about 2 to 15 mm in women of reproductive age, depending on the phase of the menstrual cycle when the ultrasound is performed. After menopause, the endometrium becomes thinner and usually measures less than 5 mm.
During the menstrual cycle, the endometrium naturally thickens and thins in response to hormonal changes as the uterus prepares for a possible pregnancy.
The normal endometrial thickness in mm is about 2 to 4 mm during menstruation. In the early proliferative phase (days 6 to 14 of the cycle), it typically measures around 5 to 7 mm, increasing to as much as 11 mm in the late proliferative or pre-ovulatory phase. During the secretory phase, the normal thickness usually ranges from 7 to 15 mm.
In women of reproductive age, an increase in endometrial thickness called endometrial hyperplasia can only be safely ruled out when the endometrium measures less than 8 mm.