According to the Mayo Clinic, a period that lasts three days is perfectly normal. Menstrual periods may be as short as two days or as long as seven and still fall within normal limits. Generally, when a woman first begins to menstruate, her periods are longer, but they become shorter and more regular with age.
The Mayo Clinic explains that a woman's menstrual cycle is defined as the time between the day when she begins one period to the day she begins the next. A typical cycle is 28 days; however, a cycle of 21 days may be normal for one woman while 35 days may be normal for the next.
According to Web MD, the length of a woman's cycle is generally less important than whether something has changed. A missed period in a sexually active woman of reproductive age often indicates pregnancy. In a woman of 40 or 50, however, it may indicate perimenopause--the period of time surrounding menopause when estrogen production begins to decrease.
Another reason for shorter or missed periods is excessive exercise. Known as exercise-induced amenorrhea, this phenomenon occurs in up to 25 percent of female athletes, and, according to the Virginia Commonwealth University Medical Center, is particularly common in women who participate in sports with a subjective component to scoring, such as gymnastics, ice-skating and ballet. Weight loss and nutritional deficiencies are common in these women as well.