Medical Health Encyclopedia Risk Factors for Shorter Cycles	Risk Factors for Longer Cycles
Regular alcohol use.	Text Continues Below Being under 21 and over 44.
Stressful jobs.	Being very thin (also at risk for short bleeding periods).
	Competitive athletics (also at risk for short bleeding periods).

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. It should be noted, however, that about 5% of healthy women menstruate less than four days and 5% menstruate more than eight 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.