Dear student,
Starting of menstruation in a woman is called menarche and the the end of menstruation is called menopause in the life cycle of a woman. So, during this period the egg would get released and can fertilize with a sperm (if copulation occurs in this period) and the chances of being pregnant and give birth to a baby is high. That is why the period between menarche and menopause is called the reproductive age.
Regards