The onset of menstruation is the bleeding caused by the stripping of the endometrium under the retreating effect of progesterone. Due to the proliferating endometrium, the thickness of the endometrium during the menstrual period is the thinnest, at around 0.3-0.5mm. After the end of the menstrual period, estrogen levels will gradually rise, the thickness of the endometrium will gradually increase, to around the day of ovulation the thickness of the endometrium will reach about 8-10mm, such a thickness of the endometrium, suitable for fertilization of the egg to bed and development. After ovulation, the level of progesterone in the body begins to rise, the endometrium growth rate slows down, and gradually undergoes changes in the secretion phase, until the next menstrual period, the thickness of the endometrium is about 10-12mm. If conception occurs after ovulation, the body’s progesterone has been maintained at a relatively high level, and the endometrium will metamorphose to accommodate the development of the fertilized egg’s implantation.