There are two main reasons for a girl to have a hard lump in her unilateral breast: 1) a local hematoma formed after trauma or impact to the breast, which can disappear after the hematoma disappears; 2) a suggestion of breast development, which can be diagnosed under ultrasound as a long breast nucleus, or long breast. Under normal circumstances, girls should start to grow breasts at the age of 11 weeks, and the development of secondary sexual characteristics will appear, which is normal sexual development. If a girl younger than 8 years old has breast growth and hard lumps, this is a sign of precocious puberty. If the breasts grow between the ages of 8 and 11 years old, the diagnosis is early sexual development. Both precocious puberty and early sexual development should be treated as soon as possible to avoid premature sexual development, which may lead to short lifelong height in the future.