Menstruation occurs due to hormonal changes in the body. The ovaries secrete the female hormones estrogen and progesterone. These hormones cause the lining of the uterus (or womb) to form. The lining is built ready for the fertilized egg to attach and begin to develop. Without a fertilized egg, the lining breaks down and bleeds. It usually takes about a month for the lining to build up, then break down. That's why most girls and women get their period once a month. If the egg is fertilized by a sperm cell, it will attach to the wall of the uterus, where, over time, it will develop into a baby. If the egg is not fertilized, the lining of the uterus breaks down, and bleeding causes a period.