The ovarian cycle starts with the follicular phase. In the follicular phase, which begins the same day your period does, the egg begins to grow and mature inside the follicle, also known as the egg sac. This phase continues until ovulation occurs, at which time you enter the luteal phase. Ovulation occurs around the fourteenth day of the entire menstrual cycle, which includes all cycles and phases. A surge of the luteinizing hormone, or LH, happens right before the egg is released from the ovary. After ovulation comes the luteal phase. At this time, the corpus luteum (Latin for "yellow body") fills the space in the ovary where the mature egg was located before its release at ovulation. The luteal phase will continue until the first day of your period.
At the same time the ovarian cycle is taking place, the uterine cycle is also occurring. This cycle consists of the proliferative phase and the secretory phase. The proliferative phase starts when your period ends, and continues until ovulation occurs again. The uterine lining grows and thickens during this time in preparation for implantation of a fertilized egg. If this implantation of a fertilized egg happens, pregnancy occurs. Your uterus begins secreting proteins after the ovulation stage, and continues to do so until your period begins. These proteins come from glands in your uterus and, again, are preparing the uterus for a fertilized egg to become implanted. If implantation of a fertilized egg does not occur, the endometrium, which is another word for the uterine lining, will begin to break down. Proteins will no longer be secreted from the glands, and the lining will begin to break away from the uterine walls. As it does so, it is passed out of your body and you have your period, also be called menstruation.
Menstruation normally lasts anywhere between three and five days, although it may be longer for some women and shorter for others. Depending on the amount of uterine lining that was formed, a period can be heavy or light. Many factors can cause changes in the uterine lining, including using oral contraceptives, which can cause lighter, shorter periods, or other physical problems, such as endometriosis, which can cause very heavy, painful periods.