Hard lumps inside the breasts of newborns are usually related to physiological factors, inflammatory lumps, endocrine abnormalities and other factors. 1. Physiological factors: it may be because in the mother’s body, affected by the mother’s estrogen level, resulting in the formation of hard lumps in the breast. Generally, after the body gradually develops, the estrogen level will return to normal, and the hard lumps will disappear gradually. 2. Inflammatory lumps: After birth, the nipple is stimulated by inflammation, which may form inflammatory edema, resulting in inflammatory lumps in the breast. 3. Endocrine abnormality: it may be due to premature gonadal development, resulting in lumps in the breast. If the hard lump inside the chest of a newborn baby doesn’t go away, you should consult a doctor in time and make a preliminary diagnosis by ultrasound, and perform a pathological biopsy if necessary.