Endometrial thickness is measured on transvaginal ultrasound as the maximum double-layer thickness of the endometrium in the sagittal plane, from the echogenic endometrial–myometrial interface on one side to the other. In premenopausal women, thickness varies with the menstrual cycle (3–4 mm in the proliferative phase, up to 16 mm in the secretory phase). In postmenopausal women with bleeding, an endometrial thickness greater than 4–5 mm warrants further evaluation for endometrial hyperplasia or carcinoma.