It may be normal for a one-year-old baby girl to have breast nuclei, but pathological conditions such as precocious puberty cannot be ruled out. If a one year old baby girl is breastfed, the estrogen in the mother’s body may be passed on to the baby through the breast milk, and the estrogen stimulates the mammary glands, which may lead to the development of breast nuclei in a baby girl, which is usually a normal condition. One year old baby girl with breast nucleus may also be due to precocious puberty, usually due to premature activation of the gonadal axis in the body, resulting in excessive secretion of estrogen in the body, which leads to the appearance of baby girl with breast nucleus. If a one year old baby girl with breast nuclei, we recommend timely medical attention, under the guidance of the doctor, complete the relevant examination, clear the cause of the disease, symptomatic treatment.