What is the reason for the enlargement of breasts in a five-month-old baby girl?

Patient's question:

Female, 5 months: Breasts have grown
Female baby, 5 months to nearly a month ago, has felt her breasts growing. They were normal before. What's going on? Very worried

Doctor's answer:

Considering the possibility that the baby is experiencing physiological early puberty, it is recommended that you first check the endocrine to confirm. If the baby is breastfed, it is suggested that you pay attention to a light and easily digestible diet for the mother. If the baby is formula-fed, it is recommended that you check if the issue lies with the formula, and switch to a different formula if necessary.
Follow-up question: The baby is breastfed. If it is physiological, how long will it last? What should I observe regularly?
Advice: It is recommended to observe for two months. It will disappear as the baby grows older. At the same time, the baby should not be too fat.