Hardness in the breasts of four-month-old babies may be caused by estrogen buildup, breast fat granules, bacterial infection and other reasons. 1. Estrogen buildup: some babies are more sensitive to estrogen, and the adrenal glands in the body will also secrete a small amount of estrogen, which under the action of estrogen may lead to four-month-old babies with hard breasts. 2. Breast fat particles: due to the four-month-old baby’s skin is relatively tender, the development of sweat and endocrine systems are not perfect, the skin secretes some oil can not be discharged in a timely manner, which may also lead to four-month-old babies have breasts with hard phenomena. 3. Bacterial infection: If there is a bacterial infection on the skin of the baby’s breast, the skin follicles may cause skin inflammation after being infected by bacteria, which may cause the four-month-old baby to develop breast nodules. In addition, transient physiological factors may also lead to the four-month-old baby breast nodules, if the four-month-old baby breast nodules phenomenon, it is recommended to go to the hospital in a timely manner, after a detailed examination, under the guidance of the doctor to actively carry out treatment.