Breast development can occur on both sides together or on one side first, and is different for each individual. Breast development may occur between the ages of 9 and 13. When the breasts are stimulated by hormones, the breast glands will increase in size, and when they first start to develop, the breasts may feel slightly swollen and painful, and the nipples will darken in color and increase in size as a result of hormones. Breasts may develop on both sides at the same time, or one side may develop first. During puberty, if both breasts have different sensitivities to estrogen, the one with higher sensitivity develops first and the one with lower sensitivity develops later, so one breast may develop first. If one side of the breast is delayed, it is recommended that parents should take their children to the hospital for examination, to clarify the cause of the disease, and under the guidance of a professional doctor for targeted treatment.