Premature breast development alone is the most common type of partial precocious puberty (also known as incomplete precocious puberty) and is commonly seen in girls. In addition to early breast development, there are no changes in the uterus or ovaries, no appearance of other sexual characteristics (e.g. pubic hair, axillary hair), and no advancement in bone age or accelerated height growth. Clinically, it is common to see female infants who are 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-infantile period” (micropubertal) and the fact that negative feedback regulation is not yet complete. Pubertal development is regulated by the hypothalamic-pituitary-gonadal axis, and the mechanism is very complex. During the perinatal and infantile 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, after 2 years of age, most of the breasts that began to increase in size during infancy gradually soften and even disappear.