When doing ultrasound examinations in the clinic, I often encounter patients who ask: “Doctor, why is my endometrium type A yesterday but type C today?” Is there something wrong? The three most common forms of endometrium under ultrasound are type A, type B and type C: Type A endometrium: commonly seen in the early stage of endometrial hyperplasia (6th-10th day of menstruation), when the thickness of the endometrium is 4-9 mm. Between the central line is a hypoechoic or dark area. Type B endometrium: Commonly seen in the late stage of endometrial hyperplasia (day 11 of menstruation – ovulation), the thickness of the endometrium is about 9-12 mm at the time of ovulation. it is a homogeneous moderate intensity echogenicity with intermittent strong echogenic midline in the uterine cavity. Endometrium type C: common in the luteal phase (i.e., after ovulation to before the next menstrual period), with a thickness of about 10-14 mm, homogeneous strong echogenicity and no midline echogenicity in the uterine cavity, when the “three-line sign” disappears. In addition, for patients who undergo ultrasound examination during menstruation, the endometrium starts to peel off and the new endometrium just starts to grow, so the thickness of endometrium <4mm is normal and there is no need to worry. Therefore, the "ABC" type often refers to the different patterns of endometrium, not a level evaluation, and the "ABC" type not only varies with menstrual cycle, but also relates to the position of the uterus, such as the median uterus, where the endometrial pattern is usually For example, in a median uterus, the endometrial pattern is usually "B" or "C". In the process of assisted reproduction, we should pay more attention to the thickness of the endometrium. If the thickness of the endometrium can reach 6mm at the time of embryo transfer, the minimum requirement for transfer can be met.