The female reproductive system performs several functions. The ovaries produce cells called eggs or oocytes. The eggs are then transported to the fallopian tube where fertilization by a sperm can occur. The fertilized egg travels to the uterus, where the uterine lining thickens in response to the normal hormones of the reproductive cycle. Once in the uterus, the fertilized egg can implant in the thickened uterine lining and continue to develop. If implantation does not occur, the uterine lining is shed as a menstrual flow. In addition, the female reproductive system produces female sex hormones that support the reproductive cycle. During menopause, the female reproductive system gradually stops producing the female hormones necessary for the functioning of the reproductive cycle. At this point, menstrual cycles may become irregular and eventually stop. One year after the cessation of the menstrual cycle, a woman is considered to be in menopause. What happens during the menstrual cycle? Women of reproductive age (starting anywhere from 11 to 16 years old) experience cycles of hormonal activity that repeat at intervals of about one month. With each cycle, a woman’s body prepares for a potential pregnancy. The average menstrual cycle lasts about 28 days and occurs in phases. These phases include: Follicular phase (development of the egg) Ovulatory phase (release of the developed egg) Luteal phase (phase during which hormone levels decrease if the egg does not implant) There are four main hormones (chemicals that stimulate or regulate the activity of cells or organs) involved in the menstrual cycle. These are follicle-stimulating hormone, luteinizing hormone, estrogen and progesterone. Follicular phase starts from the first day of menstruation. During this stage, the following events occur: Two hormones, follicle-stimulating hormone and luteinizing hormone, are released from the brain and transported to the ovaries via the blood. The hormones stimulate the growth of about 15 to 20 eggs in the ovaries, each in its own “shell” called a follicle. Follicle-stimulating and luteinizing hormone also cause an increase in the production of the female hormone estrogen. As estrogen levels rise, it shuts down the production of follicle-stimulating hormone. This careful balance of hormones allows the body to limit the number of follicles that will prepare eggs for release. As the follicular phase progresses, one follicle in one ovary becomes dominant and continues to mature. This dominant follicle suppresses all other follicles in the group. As a result, they stop growing and die. The dominant follicle continues to produce estrogen. The ovulatory phase usually begins about 14 days after the start of the follicular phase, but it can vary. Most women have a menstrual cycle 10 to 16 days after ovulation.During this phase it follows: The increase in estrogen from the dominant follicle causes a spike in the amount of luteinizing hormone that is produced by the brain. This causes the dominant follicle to release its egg from the ovary. As the egg is released (a process called ovulation), it is captured by protrusions at the end of the fallopian tubes (fimbriae). For one to five days before ovulation, many women notice an increase in cervical mucus. This is the vaginal secretion that helps trap and nourish sperm on their way to the egg. The luteal phase begins immediately after ovulation. After releasing its egg, the empty ovarian follicle develops into a new structure called the corpus luteum. The corpus luteum secretes the hormones estrogen and progesterone. Progesterone prepares the uterus for implantation of the fertilized egg. If intercourse has taken place and the egg is fertilized by a sperm, the fertilized egg (embryo) will travel through the fallopian tube to implant in the uterus. If the egg is not fertilized, it passes through the uterus. It is not needed to maintain pregnancy, the lining of the uterus breaks down and is shed, the next menstruation begins. Source: my.clevelandclinic.org