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The Menstrual Cycle

The menstrual cycle, about 28 days, varies from woman to woman, and can vary during a woman's life.

Cycle Begins (Follicular Phase)
Graphic of the hypothalamus 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.
Early Days
Graphic depicting endometrial lining 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.

Graphic depicting the menstrual cycle Near Midcycle
Graphic of a follicle rupture 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.

Fertile Time
Graphic of an unfertilized egg 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.

Late Cycle
Graphic of a pregnant woman 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.
Near Midcycle (Luteal Phase)
Graphic of an ovary 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.

Sources:

Hatcher RA, Trussell J, Stewart F et al. Contraceptive Technology, Sixteenth Revised Edition. New York: Irvington Publishers, 1994.
Fertility Awareness: An Important and Effective Approach to Family Planning and Reproductive Health. Slide presentation. Washington: The Institute for Reproductive Health, Georgetown University, 1996.
Harlow SD, Ephross SA. Epidemiology of menstruation and its relevance to women's health. Epidemiol Rev 1996;17(2):265- 86.

Cycle Begins (Follicular Phase) 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.	Early Days 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.
	Near Midcycle 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.
	Fertile Time 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.
Late Cycle 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.	Near Midcycle (Luteal Phase) 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.