During puberty, a girl's body begins to resemble that of a woman. Her hips grow wider, and her breasts grow bigger. 

During puberty, the egg cells in the girls ovaries begin to mature. Once a month, one of these egg cells develops and is detached from the ovary. The egg cell travels to the womb through the Fallopian tube. 

The mucous membrane of the womb has grown in size, which means that it is ready to receive a fertilized egg cell. If a male sperm cell does not fertilize a female sex cell during this time, the egg cell will not attach to the womb's mucous membrane. When this happens, the mucous membrane is released from the vagina in the form of menstrual flow. For some girls, they can be painful. 

The menstrual cycle of a pubescent girl can be irrelugar, but they will become more regular as the girl develops into a woman. In adult women, the menstrual period occurs approximately every 28 days. The menstrual flow stage usually only lasts for a couple of days.