Endometrial thickness of 5.5mm is normal. The thickness of the endometrium varies during different cycles of menstruation. The endometrium is thinnest after menstruation, only 1-2mm, after which it thickens. During the proliferative phase which is day 6-14 of the menstrual cycle, the thickness can reach 1-3mm, during the secretory phase which is day 15-28 of the menstrual cycle, the thickness can reach 5-7mm, and just before the onset of menstruation, the endometrium reaches its thickest, at around 10mm.