Premenstrual Syndrome - Introduction

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Stages and Features of Menstruation

Onset of Menstruation (Menarche). Previous evidence had set the onset of menstruation, called the menarche, at an average of age 12 or 13. Recent studies, however, set the time of onset earlier by about 1 year in Caucasian girls and 2 years in African American girls. Currently, the youngest possible age for normal puberty is 7 years old for Caucasians and 6 years old for African Americans, down from a previous low of 8 years for both.

Evidence suggests that the increasing incidence of childhood obesity is a major cause of the trend in earlier menarche onset. (Obesity is also highly associated with hormonal disorders in girls entering puberty at young ages.) Environmental estrogens found in chemicals and pesticides are also suspects.

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Length of Monthly Cycle. The menstrual cycle can be very irregular for the first 1 or 2 years, usually being longer than the average of 28 days. The length then generally stabilizes to an average of 28 days, although the cycle length may range from 20 to 45 days and still be considered normal. A variation of 10 days or more--either more or fewer days--may have an impact on fertility, however. When a woman reaches her 40s the cycle lengthens, reaching an average of 31 days by age 49. A number of factors can affect cycle length at any age.

Length of Periods. Periods average 6.6 days in young girls. By the age of 21, menstrual bleeding averages 6 days until women approach menopause. However, about 5% of healthy women menstruate less than 4 days and 5% menstruate more than 8 days.

Normal Absence of Menstruation. Normal absence of periods can occur in any woman under the following circumstances:

• Menstruation stops during the duration of pregnancy. Some women continue to have irregular bleeding during the first trimester. This bleeding may indicate a threatened miscarriage and requires immediate attention by the doctor.
• When women breastfeed they are unlikely to ovulate. 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.

Review Date: 07/11/2006
Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital.

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