The Normal Menstrual Cycle

Prior to having her first episode of menstrual bleeding (menarche), a maturing girl will show signs of puberty beginning with breast development. Next are the appearance of pubic hair, growth spurts and armpit hair. The actual occurrence of menstrual bleeding is usually the last event in an orderly progression of changes. In America, a girl will experience menarche between the ages of 9 and 16 years, with the average age being 12 years. During the first few years of the menstrual cycle, ovulation may not occur with each episode of menstrual bleeding. Ovulation is the process by which an unfertilized egg leaves the ovary for passage into the fallopian tube.

How does menstruation happen?
Through a complex coordinated mechanism that involves hormone production in the brain and ovaries, three organs (brain, ovaries and uterus) work together to produce female sexual maturity and the woman's ability to have a baby.

The length of menstrual cycles can vary quite a bit, but the average length is 28 days (range 26 to 30 days). Your cycle is divided into two parts separated by ovulation. These parts are the follicular phase and the luteal phase.

Follicular Phase of the Menstrual Cycle
In the average cycle, the follicular phase lasts about 10 to 14 days before ovulation. This is the time that the hormonal mechanism prepares the ovaries to release an unfertilized egg (oocyte) and causes the lining of the uterus (endometrium) to thicken. The main hormone in this phase is follicular stimulating hormone.

The endometrium grows and becomes thickened with blood vessels, glands and other tissue. While this process is happening, several unfertilized eggs are growing and maturing in the ovaries. These eggs develop inside fluid-filled sacs called follicles. Usually one follicle will grow faster than all the others. Occasionally, two may mature at the same time.

Throughout the menstrual cycle, glands in the cervix produce mucus. Female hormones affect the amount of and quality of cervical mucus appearing at the vaginal opening. During the follicular phase, the glands begin to increase mucous production and the cervical opening begins to widen.

Ovulation
When the fastest growing follicle reaches maturity, it breaks open and releases the egg (oocyte) from the ovary. Upon release, the oocyte enters the fallopian tube and travels towards the uterus. Several hormones play an important role in ovulation, including but not limited to, estrogen, lutenizing hormone, and progesterone. Sometimes, a woman may get pelvic pain when the rupture of the oocyte from the ovary happens. This is call mittelschmerz. Also, there may be vaginal spotting at the time of ovulation. [CAUTION: This spotting is not the beginning of your menstrual bleeding.]

During ovulation, the cervical mucus tends to be most abundant and thinnest, "stringy." The thin mucous then flows down through the vaginal opening. The days immediately surrounding ovulation are the most likely times that a pregnancy might occur if sperm are present. This would be a woman's most fertile time.

The Luteal Phase of the Menstrual Cycle This phase begins approximately 14 days after ovulation, and lasts about 14 days in the average 28-day cycle. The length of this phase does not vary as much as that of the follicular phase.

Under the influence of hormones, the fluid-filled sac that released the egg at ovulation, develops into a structure called the corpus luteum. The corpus luteum produces a hormone called progesterone to help keep the thickened uterine lining (endometrium) in place for a possible pregnancy. If no pregnancy occurs, the corpus luteum stops producing progesterone and disintegrates. Then, the thickened uterine lining is shed as menstrual bleeding.

Menstrual Bleeding
The first day of menstrual bleeding is counted as DAY 1 of your menstrual cycle. Menstrual bleeding, also called the menstrual period, usually lasting about 3 to 5 days. Each woman should keep a personal record of her monthly menstrual cycle and put a big "x" on the days of bleeding.

Sources:
Scott JR, Di Saia PJ, Hammond CB, Spellacy WN. Danforth's Obstetrics & Gynecology, 8th Edition. Philadelphia: Lippincott Williams & Wilkins, 1999.

Heffner LJ. Human Reproduction at a Glance. Malden, ME: Blackwell Science LTD, 2001.

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