Some newborns, especially baby girls, develop hard lumps in their breasts shortly after birth, usually due to interruption of maternal estrogen. Most of these lumps are temporary and will subside after 2-3 weeks of life. If the lump does not subside after 4 weeks, parents can take their child to the hospital. This is because both male and female newborns can experience temporary enlargement of the mammary glands around 4-7 days after birth, and most of the breast lumps are the size of a fava bean or a walnut. Parents do not need to worry too much about this situation and should not squeeze it, as improper squeezing can cause localized infection or even sepsis.