The sexual development of children generally starts after eight weeks of age for girls and nine weeks of age for boys. In the early stage, girls can show small nodules in the nipple area, the sensitivity of each child is different, some girls will feel localized soreness and swelling, or even slight tenderness, later the breast gradually increases, the discomfort will gradually disappear. At the same time, the uterus and ovaries also begin to develop, the endometrium thickens and the follicles gradually increase in size. It normally takes about 4-5 years from sexual initiation to menstruation, so it should be normal for a girl to have her period at the age of 12.