An important change during adolescence for a female, is the beginning of a monthly cycle to prepare the uterus to receive and nourish a possible fertilized egg. this monthly cycle is called the menstrual cycle. ap- proximately every days, an egg is released from one of the and begins to travel down the the alternate the release of eggs. one month, the left releases an egg, and the next month, the right takes its turn. the release of an egg from an ovary is referred to as during this time, the has been undergoing changes. the walls of the fill with and become thicker. if the egg that travels through the becomes fertilized by a then the egg will implant itself in the lining of the and begin the process of devel- opment, which can lead to new life. if the egg isn't it will break down and leave the body with the used and tissue that had collected in the wall. the blood and tissue leave the body through the over an average -day period of time. this is called