A menstrual cycle begins from the first day of the period and ends when the next period begins. Hormone signals are sent between the brain and the ovaries, causing changes in the ovaries that contain the eggs. The first phase of the menstrual cycle prepares an egg to be released from the ovary and builds the lining of the uterus. The second phase prepares the uterus and body to accept a fertilized egg during pregnancy or to start the next menstrual cycle if pregnancy doesn’t happen.