Simple premature breast development is one of the most common types of partial precocious puberty. In such a baby girl/girl, apart from early breast development, there are no changes such as enlarged uterus, early maturation of follicles, and no manifestations such as appearance of pubic and axillary hair, arrival of first menstruation, early bone age and accelerated height growth. Recently, we have encountered several cases of several months old female infants with enlarged breasts, which were diagnosed as simple premature breast development, and the parents of the children were extremely worried. Why does early development of simple breasts occur in infancy? This may be related to the fact that the hypothalamic-pituitary-gonadal axis is temporarily activated during infancy, and the corresponding negative feedback regulation in the body is not yet sound. In infancy, the hypothalamic-pituitary-gonadal axis has a low sensitivity to negative feedback, so when there is a slight external stimulus, such as bad mood or bad sleep, there will be a temporary fluctuation of specific sex hormones (e.g. estrogen, E2), but the hypothalamic-pituitary-gonadal axis will not respond to it, so there will be an increase of estrogen and no significant decrease of follicle stimulating hormone (FSH) secretion released by the pituitary gland. This results in a transient increase in both E2 and FSH in the blood, leading to breast enlargement in female infants. After the age of 2 years and before the onset of puberty, the hypothalamic-pituitary-gonadal axis gradually enters a resting phase in girls, so that the breast enlargement that begins in most girls after the age of 2 years gradually softens and even disappears. This kind of performance in infancy and early childhood is medically referred to as “micro-puberty”, which is pseudo-precocious rather than true precocious puberty. The early breast development alone can also occur in childhood. In addition to the fluctuation and instability of the hypothalamic-pituitary-gonadal axis, it may also be related to the intake of food containing sex hormones (such as colostrum, pigeons, eels, etc.) and frequent exposure to pictures or videos related to “sex” in the environment. However, at this age, the hypothalamus-pituitary-gonadal axis is not fully activated. Therefore, it is necessary to avoid the stimulation of external sex hormones, strengthen the child’s psychological counseling, and establish a correct understanding of “sex”. Simple premature breast development does not have a negative impact on the growth and development of the child, so no special treatment is needed. However, parents must be aware that some children diagnosed with “simple premature breast development” may have other underlying causes, such as McCune-Albright syndrome (MAS). If this cause is not detected and removed in a timely manner, the breasts may continue to increase in size and may develop into true precocious puberty. If true precocious puberty occurs, it will eventually affect the child’s height and physical and mental health. Therefore, regular follow-up, review of the child’s bone age, uterine and ovarian/testicular ultrasound, and examination of sex hormone and gonadotropin-releasing hormone levels can help detect true precocious puberty early, which is beneficial for the child’s eventual height enhancement and the child’s physical and mental health.