Many of you have obtained high quality embryos during the cycle, and the next step is embryo transfer, but before the transfer, you always hear that you need to transform the endometrium. In fact, the so-called transformation of the endometrium is the conversion of the endometrium from the proliferative phase to the secretory phase through the action of progesterone, which is commonly known as fertilizing the soil of the endometrium to make it easier for the embryos to take root and grow. In the natural state of the menstrual cycle, there is an “implantation window” for the endometrium, which usually lasts for about 24 hours during the 20-24 days of the menstrual cycle. This means that during this particular period, the endometrium is ready to receive the embryo for implantation, which corresponds to the optimal implantation state during this period. Thus, the endometrial transformation we do is to regulate the endometrium to this optimal state by artificially regulating it, simulating the natural state. In our cycle therapy, the common types of endometrial transformation can be divided into artificial cycles and natural cycles. In the artificial cycle, progesterone is added when the endometrial thickness reaches 8mm or more. In natural cycles, progesterone is added after ovulation is monitored by ultrasound. Adequate levels of progesterone are the basis for endometrial transformation and the opening of the “implantation window”, as well as the key to stable early embryo growth. Before the transformation of the endometrium, we can observe the status of the endometrium by ultrasound. When the thickness, morphology and blood flow of the endometrium reach a good level, we can start the treatment of transformation of the endometrium. After the endometrial transformation, the uterus will have the basic conditions for the growth of the embryos, and the next step is the embryo transfer.