Endometrial lining (lining the uterus) sheds. Hypothalamus, a region of the brain, produces gonadotropin-releasing hormone (GnRH), which is necessary for the release of follicle-stimulating hormone (FSH) from the pituitary gland.

FSH causes small follicles to develop inside the ovary. One follicle begins to grow larger and secretes estrogen, a female hormone. This dominant follicle produces a cell (an oocyte) that will become an ovum, or egg.

Within days after menstrual bleeding begins, the endometrial lining begins to rebuild, stimulated by increased production of estrogen. In the early part of the menstrual cycle, a woman may notice thick and sticky cervical mucus. As estrogen increases, mucus becomes thin, watery and clear. After ovulation, the mucus again becomes cloudy and thick.

Around midcycle, increasing estrogen levels stimulate secretion of GnRH and another hormone from the pituitary gland, the luteinizing hormone (LH). This LH hormone, in turn, triggers the release of the egg from the ovary (ovulation), and stimulates the production of a second female hormone, progesterone. Ovulation occurs within 12 hours after LH production reaches its peak. As LH peaks, estrogen levels temporarily drop, and this can cause midcycle bleeding for some women. As the ovarian follicle ruptures, some women experience abdominal pain.

An unfertilized egg lives only up to 24 hours after ovulation. However, sperm cells can live as long as five days in the female reproductive tract. If sperm are present as the egg travels from the ovary through the fallopian tube, fertilization can occur.

If the egg is fertilized, production of estrogen and progesterone continues and the endometrial lining becomes even thicker. The fertilized egg may become implanted in the endometrial lining. The endometrial lining does not shed, and the pregnant woman's menstrual period does not begin.

If fertilization does not occur, estrogen and progesterone levels fall, the endometrial lining sheds, and menstrual bleeding begins again.

The cycle, about 28 days, varies from women to woman, and can vary during the same women's life.

The ruptured follicle, now called the corpus luteum, begins to produce estrogen and progesterone. Increased amounts of estrogen and progesterone cause a decrease in FSH and LH and cause a change in the endometrial lining. The lining becomes thicker and its blood supply increases, preparing the lining to accept an embryo if fertilization and implantation occur.