While books often portray the menstrual cycle as a perfect 28 days, in real life there is much variation, both from woman to woman and for the individual woman over her life span. Medically, normal cycle length (the time between the first day of bleeding in one cycle and the first day of bleeding in the next) is anywhere from 21 to 35 days. In the first one to two years of menstruation it is quite frequent for menses to be irregular. After that time a woman, and especially one who is keeping track of her onot perishah (times of separation) knows approximately when to expect her period. Uterine bleeding outside the normal cycle times for most women, or markedly different from the usual pattern of a particular woman, is known as menstrual irregularity.