An endometrial lining thickness of 4.4mm is a normal phenomenon, and the thickness of the endometrial lining varies with the menstrual cycle. The endometrial lining is 4.4mm thick, usually at the very end of the menstrual cycle or during menstruation, when the endometrial lining is at its thinnest. As the menstrual cycle ends, the endometrium begins to gradually thicken due to hormone levels. Before menstruation, during the secretory phase, the lining of the uterus may reach 10-18mm, so the thickness of the lining varies with the menstrual cycle. If the endometrium is 4.4mm before menstruation, then the endometrium is too thin. A thin endometrium may cause infertility, or a low menstrual flow, which would need to be treated clinically.