The causes of lumps in one breast of a 6-week-old girl may be due to precocious puberty, subcutaneous nodules or lipoma. 1. Precocious puberty: due to the stimulation of the secretion of sex hormones, the girl may lead to the appearance of breast hard lumps in the breast, and may also have a slight pain, which may be related to the long-term consumption of high-calorie and hormone-containing food triggered by the disruption of the endocrine system. 2. Subcutaneous nodule: if the girl’s breast is squeezed and rubbed by external force for a long time, it may suffer from chronic inflammation, leading to tissue proliferation, which will also cause lumps in the breast. 3. Lipoma: Lipoma is a kind of soft tissue benign tumor, which will cause abnormal proliferation of fat tissue under the skin, and if the location of the lipoma is in the chest, it will appear chest nodules. In addition, chest wall diseases and breast trauma and other factors may also cause 6-year-old girls to develop breast lumps. When parents find that their children’s breasts have lumps, they should immediately consult a doctor to find out the cause and cooperate with the doctor’s treatment.