There are two cycles involved in the regulation of a woman's fertility: the ovarian cycle and the uterine cycle (also called the menstrual cycle). The ovarian cycle is one of the two cycles that regulates a woman's fertility. The events in the ovary that produce an egg each month represent the ovarian cycle. The uterine cycle represents a sequence of changes in the uterine lining (endometrium) in response to ovarian hormones. Phases include menstruation, rebuilding of the endometrium, and preparation for implantation of an embryo. The ovarian and uterine cycles are controlled by changes in hormone levels. The action of gonadotropin-releasing hormone (GnRH) induces the release of other hormones, including follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland and the estrogen hormones (the main hormone of which is estradiol) and progesterone from the ovaries. Although these hormones have effects across multiple systems, their primary roles are to regulate a woman's fertility.
The first phase of the ovarian cycle, known as the follicular phase, produces a dominant follicle and ends in ovulation. More specifically, the ovarian cycle begins with a rise in FSH levels, causing a small group of primary follicles (small follicles containing primary oocytes—diploid cells that will eventually give rise to a mature ovum) to begin to grow. Approximately 10 of these Graafian follicles begin to mature, until all but one die, leaving a single Graafian follicle, which is the preovulatory follicle. At this point, levels of estradiol drop and levels of LH rise. High levels of LH cause the follicle to rupture, releasing the egg from the ovary. This process is called ovulation. The follicular phase is about 14 days long.The second phase of the ovarian cycle is the luteal phase, and it also lasts about 14 days. The oocyte is triggered to continue meiosis, while the cells that made up the Graafian follicle form a small clump called the corpus luteum. The corpus luteum produces progesterone, a hormone that prepares the uterus for implantation of a zygote. The zygote is the fertilized ovum, or egg, with diploid (2n) number of chromosomes. At this point, FSH and LH levels drop quickly. If fertilization does not occur, then the corpus luteum degenerates. If fertilization does occur, hormones associated with pregnancy maintain the corpus luteum. The decline of progesterone toward the end of the ovarian cycle triggers the beginning of the uterine (menstrual) cycle. If fertilization does not occur, menstruation begins. During menstruation, the thick lining of the uterus sloughs away, along with blood and mucus. The term sloughing means that there is death of the cells of the uterine lining, and the lining is subsequently shed in layers. This period lasts between two and seven days for most women. During this time, women may experience other effects, such as uterine cramping or breast tenderness. Proliferation, the next phase of the uterine cycle, begins following menstruation. Proliferation is characterized by a thickening of the uterine lining in response to increasing levels of estrogen. An estrogen is a hormone responsible for many functions in the female body, including the promotion of the growth of the endometrial lining, inhibition of bone resorption, decreasing the risk of heart disease, and affecting fluid/electrolyte levels. Estrogen increases blood levels of high-density lipoprotein (HDL) and decreases low-density lipoprotein (LDL) levels, thereby decreasing the risk of heart disease. Estrogen affects fluid and electrolyte balance by increasing sodium and water retention. Estrogen can affect a woman's mental health because it can increase serotonin levels, a neurotransmitter thought to contribute to feelings of well-being. In the final secretory phase of the uterine cycle, the lining of the uterus is prepared for implantation of the zygote. The secretory phase is triggered by the production of progesterone by the corpus luteum, which causes the uterine lining to become smooth through an increase in blood flow and glandular secretions.
Contraceptives, such as birth control pills, rings, and implants, can be used to disrupt the ovarian cycle and prevent pregnancy. These contraceptives work by altering the production of FSH and LH, thereby inhibiting the growth of follicles and subsequent ovulation.