Whether endometrial thickness of 0.4 is normal or not depends on which cycle the endometrium is in. Endometrial proliferation is divided into proliferative, secretory and shedding phases, and the thickness of each cycle is different. If the endometrium is in the early stage of hyperplasia, it is normal. If it is in the middle or late stage of hyperplasia, or in the shedding stage, it is thin, and it is recommended to actively treat it. The thickness of the endometrium varies with physiological changes. Under the influence of estrogen, endometrial growth is divided into proliferative, secretory and shedding phases, and the shedding phase is also known as the menstrual phase, with varying thicknesses of the endometrium during each cycle. The thickness of the endometrium in the early proliferative stage is about 4~6 millimeters, in the middle proliferative stage is about 8~10 millimeters, and in the late proliferative stage is about 9~10 millimeters; the thickness of the endometrium in the early secretory stage is basically the same as in the middle stage, and in the late stage it is about 5~6 millimeters; and the thickness of the endometrium in the exfoliative stage is about 8~10 millimeters. The thickness of the endometrium changes with the physiological cycle. It is best to start the examination on the third day after the menstrual period is cleaned, when the endometrium is thinner, less prone to bleeding, less secretion, and the image is clear. If the thickness of the endometrium does not reach 8mm during menstruation, it suggests that the endometrium is thin and may affect conception. If you have any other questions, it is recommended that you consult a medical professional for guidance.