The ovary in females produces female gametes called eggs. Each ovary releases an egg every month alternatively. After the egg is released, it moves into the fallopian tube where it stays for about 24 hours to be fertilized by a sperm. Also, at the same time, the uterus prepares itself by forming a thick and spongy lining to receive the embryo and nourish it. If the egg is not fertilized during this time, the lining of the uterus (endometrium) is shed along with blood and mucus; this process is called menstruation. The whole event is a 28-day long cycle on an average and is called menstrual cycle.