There are many reasons why one year old babies have hard lumps in their breasts, such as localized skin infections of the mammary glands, excessive estrogen, and cystic lumps in the mammary glands. 1. local skin infection of the mammary gland: the local skin of the mammary gland distributes many hair follicles, if the hair follicles are infected by bacteria, especially Staphylococcus aureus, can cause folliculitis, resulting in the phenomenon of one year old baby breast lumps, and in severe cases, pus can be formed. 2. Too much estrogen: too much estrogen in the baby’s body can lead to temporary enlargement of the mammary glands, may be due to the mother’s excessive estrogen at birth, resulting in excessive estrogen in the baby’s body after birth, usually after the estrogen is lowered, it will be gradually relieved. 3. Breast cystic lumps: one year old baby breast lumps may belong to dermoid cysts, most of the congenital development, mostly painless lumps. There are many reasons why a one year old baby has a hard lump in the breast, and it is recommended that parents take their babies to the doctor in time to get a clear diagnosis and standardized treatment, and not to ignore the symptoms and take them lightly.