Hard lumps in unilateral breasts in little girls depends on how old a little girl she is. Because the newborn baby due to the influence of the mother’s estrogen, may be after the birth of the breast enlargement, there are most of the bilateral enlargement, there are also some unilateral breast enlargement. Generally after a period of time, this breast enlargement will disappear as the influence of mother’s estrogen weakens, this is a normal state, do not need too much intervention, especially pay attention to do not squeeze in order to avoid triggering infections. 5-6 years old children occasionally appear on one side of the breast with hard lumps, this time is the possibility of breast nucleus is more likely to be, that is, the mammary gland appears to have a small nodule, usually with the endocrine disorders or external factors. It is usually related to endocrine disorders or the influence of external estrogen, and some children may disappear without special intervention. In some children around 8 years old, if there is a hard lump on one side of the breast, it is necessary to pay attention to whether there is a tendency of development, but the general development is mainly symmetrical bilaterally, but also from one side. For this kind of children, if there is a hard lump on one side of the breast and there is a tendency to increase in size, it is also necessary to pay attention to some less common like breast hyperplasia, tumor, abnormal development of the breast, although it is not very common in the little girl, but it is not unheard of. If there is an obvious tendency to increase in size, the child should be sent to the hospital to do breast ultrasound or some other examination, to be able to clarify what the cause of the child’s condition is in the end.