It is the most common type of partial precocious puberty (also known as incomplete precocious puberty) in girls; in addition to early breast development, it is not accompanied by changes in the uterus and ovaries, no other sexual characteristics (e.g., pubic hair, axillary hair), and no early bone age or accelerated height growth. Clinically, it is common to see female infants a few months old diagnosed with simple premature breast development. These infants are both formula- and milk-fed and breastfed. Why do infants develop premature simple breast development? It may be related to the temporary activation of the hypothalamic-pituitary-gonadal axis during the “perinatal-infant period” (micropubertal) and the fact that negative feedback regulation is not yet in place. The development of puberty is regulated by the hypothalamic-pituitary-gonadal axis, and the mechanism is very complex. Generally, there are four stages of sex characteristics development: fetal, perinatal-infant, childhood, peri-pubertal and adolescent. It has been found that during the perinatal and infant periods, 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 transient 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, most breasts that began to enlarge in infancy after the age of 2 years gradually soften or even disappear. 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 development of simple breasts in childhood may be associated with 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 alone. The most important thing in dealing with premature breast development is to exclude true (central) and pseudo (peripheral) precocious puberty. Premature breast development alone does not require excessive treatment. However, avoiding as much as possible the stimulation of harmful factors and intensive follow-up are needed. Parents must be aware that some cases of simple premature breast development may turn into true precocious puberty. If the breasts do not subside or continue to increase in size, a follow-up visit is needed to review the left hand bone film, uterine and ovarian ultrasound, and if necessary, LHRH stimulation test for timely detection and treatment.