The menstrual cycle reflects the changes that occur in the endometrium -- the membrane lining the uterus. Layers of the endometrium are shed at the end of the cycle as part of menstrual flow. The menstrual cycle is generally divided into phases: the follicular (or proliferative) phase; ovulation; and the luteal (or secretory) phase.

Follicular (Proliferative) Phase

The follicular phase includes menstrual bleeding followed by proliferation -- growth and thickening -- of the endometrium. It usually lasts 10 to 14 days. For a clear picture of the process it is important to understand how to count days in a menstrual cycle. Day one is generally considered to be the first day of bleeding. The menstrual flow lasts an average of six days. Estrogen and progesterone levels are at their lowest during this time. At the end of the menstrual flow, the proliferative phase begins and the endometrium starts to grow and thicken. FSH levels rise and stimulate several ovarian follicles to mature over a two-week period until their eggs nearly triple in size. During this period, FSH also signals the ovaries to produce estrogen, which in turn, stimulates a great surge of LH around day 14. This surge of LH then triggers ovulation by causing the largest follicle to burst and release its egg into one of the two fallopian tubes.

Ovulation and Secretory (Luteal) Phase

At ovulation (usually day 14 in a 28-day cycle), the proliferative stage ends and the secretory (luteal) phase begins, which lasts about 14 days. This is also commonly known as the premenstrual period. Once ovulation has occurred, LH causes the ruptured follicle to develop into the corpus luteum, a mound of yellow tissue that produces progesterone. Acting together, progesterone and estrogen stimulate the tissue lining the uterus to prepare a thick blanket of blood vessels where a fertilized egg can attach and develop. If an egg is fertilized, this blood-vessel blanket develops into the placenta. The corpus luteum continues to produce progesterone and estrogen. When fertilization does not occur, the corpus luteum degenerates to a form called the corpus albicans (Latin for "white body"), and estrogen and progesterone levels drop. Finally, the endometrial lining sloughs off and is shed during menstruation.

Menarche

The onset of menstruation, called the menarche, usually occurs at 12 or 13 years. A recent study, however, has indicated that girls may be starting puberty earlier than in previous times. By the age of eight, 48% of African-American girls and 15% of white girls were showing pubic hair and developing breast buds. It was previously thought that only 1% of girls exhibited such changes at that age. Being overweight is a risk factor for early puberty, and the increasing trend toward early obesity in the U.S. may play a role in the decreasing age of menarche. Some experts believe, however, that this trend may be due to environmental estrogens found in chemicals and pesticides. They are especially concerned about hair products that contain estrogens, which are being used by some young girls.

Length of Monthly Cycle

Menstruation can be very irregular for the first one or two years. Then the cycle stabilizes and averages about 28 days. The cycle, however, may range from 20 to 40 days and still be normal. Age is a major factor in cycle variation. In one long-term study of midwestern women, the monthly cycle averaged 33 days for women under 21; 28 days after age 21; and 26 days by age 40. Cycle time lengthened by age 46, reaching an average of 31 days by age 49. Thin women and athletes tend to have longer cycles. Women who drink alcohol regularly tend to have shorter cycles.

Length of Periods

Periods average 6.6 days in young girls; by the age of 21, menstrual bleeding averages six days until women approach menopause. It should be noted, however that about 5% of healthy women menstruate less than 4 days and 5% menstruate more than 8 days. Thin women, particularly those who smoke, tend to have longer periods, while athletes tend to have shorter ones. Women who use oral or injected contraceptives generally have longer periods.

Cessation of Menstruation

During pregnancy menstruation stops, of course. Some women continue to have irregular bleeding during the first trimester. This bleeding may indicate a threatened miscarriage and requires immediate attention by the physician. When women breast feed they are unlikely to ovulate during the first eight weeks after delivery. After that time, menstruation usually resumes and they are fertile again.

Perimenopause starts when the intervals between periods begin to lengthen, and it ends with menopause itself (the complete cessation of menstruation). Menopause usually occurs at about age 51, although smokers often go through menopause earlier.