What is simple premature breast development? Premature breast development alone is the most common type of partial precocious puberty (also known as incomplete precocious puberty) in girls; in addition to early breast development, there are no changes in the uterus or ovaries, no other sexual characteristics (such as pubic hair or axillary hair), and no early bone age or accelerated height growth. Why does early development of simple breasts occur in infants? It may be related to the temporary activation of the hypothalamic-pituitary-gonadal axis during the perinatal-infant period (microadolescence) and the fact that negative feedback regulation is not yet complete. It has been found that during the perinatal and infant period, the hypothalamic-pituitary-gonadal axis has low negative feedback sensitivity, and when the ovarian secretion of estrogen (E2) increases, the secretion of pituitary follicle-stimulating hormone (FSH) does not decrease significantly, resulting in a temporary increase in both E2 and FSH in the blood, leading to breast enlargement. Perinatal and infantile periods are also particularly susceptible to transient breast enlargement due to the influence of external factors. After 2 years of age and before the onset of puberty, the hypothalamic-pituitary-gonadal axis enters a resting phase in girls, during which the hypothalamic-pituitary-gonadal axis is normally highly sensitive to negative feedback from sex hormones. Therefore, after the age of 2 years, most of the breasts that started to increase in size during infancy gradually become softer and even disappear. Of course, infantile breast development also requires the exclusion of pathological factors. Premature breast development alone can also occur in childhood (before the age of 7 to 8 years), and the mechanism of occurrence is not identical to that of infancy. In addition to the instability of the hypothalamic-pituitary-gonadal axis, the occurrence of premature simple breast development in childhood may be associated with the following factors: a long-term high-protein diet, the influence of estrogen-like pollutants in the environment, the intake of foods containing sex hormones, and frequent exposure to sex-related media. What is clear is that the hypothalamic-pituitary-gonadal axis is not yet fully activated in cases of premature breast development. What is the correct way to treat early breast development? The most important thing to do is to rule out true (central) and pseudo (peripheral) precocious breast development. Premature breast development alone does not require excessive treatment. However, avoidance of stimulation by harmful factors and intensive follow-up are needed. Parents must be aware that some cases of simple precocious breast development may turn into true precocious puberty. If the breast does not subside or continues to increase in size, a follow-up visit is needed to review the left hand bone film, uterine and ovarian ultrasound, and LHRH stimulation test if necessary, for timely detection and treatment.