Parents often find that their babies’ breasts are enlarged and have hard nodes at a few months old. In fact, parents do not have to worry too much, this situation is medically known as “micro puberty”, this period of negative feedback between the hypothalamus – pituitary – gonadal axis in the newborn and early infant has been established, but its inhibitory function is not yet mature. At this time, gonadotropins are secreted in a high state to reach puberty level, so it is called “micro puberty”, which often occurs in children under 2 years old. 1. Why is it more likely to occur in infants under 2 years old? During fetal life, the hypothalamic-pituitary-gonadal (testicular or ovarian) axis exists as one of the important endocrine regulatory systems in the body, but it is temporarily put on hold because of the large amount of estrogen produced by the placenta. After birth, as the umbilical cord is cut and the fetus loses its connection to the mother, the fetal endocrine system must begin to learn to carry the burden alone, but the organ that controls the development of human sexual characteristics (the hypothalamic pituitary gonadal axis) is not yet well developed, and some infants with an active gonadal axis are prone to an increase in estrogen and follicle-producing hormone in the body, which leads to breast enlargement. So we can think of micro-puberty as a small preview of the real “big” puberty a few years later. The actual meaning of micro-puberty is not well understood and its manifestations vary from person to person, so it is often overlooked, but it is indeed a physiological phenomenon that exists early in our lives. 2. What are the clinical manifestations of these children? Female infants often show enlargement of the breasts, simple premature breast development, which is the most common type of partial precocious puberty, commonly seen in girls; in addition to early breast development, it is not accompanied by changes in the uterus and ovaries, no other sexual characteristics, and no growth acceleration or early bone age. Clinically, it is common to see female infants who are a few months old diagnosed with simple premature breast development. Due to fluctuations in estrogen levels, even rare female infants may experience a little bleeding similar to menstruation in adolescent girls, but the amount of blood is usually small and of short duration. These manifestations are usually short-lived and not very obvious, but can occur before the age of 2 years. This manifestation in infancy and early childhood is extremely similar to the process of true pubertal development. Male infants often show mild testicular enlargement, penile erection, and even a transient manifestation of a little facial acne that lasts until about half an age. 3. What should parents pay attention to in these children? Most children will heal on their own after the age of two, so parents can rest assured. However, there are some children who do not heal on their own. If the child’s breasts continue to increase in size after the age of 2, it is important to pay attention to them, as this may lead to true precocious puberty, and, on the other hand, if the child has persistent vaginal bleeding or overflow of milk during the follow-up, parents should promptly take these children to a pediatric endocrinologist.