The causes of hard lumps in the breasts of newborns include the following: 1. Due to the physiological phenomenon of newborns, the hormone levels in the body of newborns are different from the hormone levels in the mother’s womb, thus causing metabolic disorders in the hormone levels in the body and leading to the symptoms of hard lumps in the breasts of newborns. This is a normal physiological phenomenon that will naturally ease as the newborn gets older and parents do not need special treatment. 2, caused by newborns with breast tumors, if the newborn has a breast tumor will also cause hard lumps in the breast, and the tumor may gradually increase and will not slowly get better, to give If the newborn has breast tumor, it will also cause breast lumps, and the tumor may increase gradually and will not get better.