chapter  2
Physiology of menstruation
ByMichael Zinger
Pages 6

The start of menses heralds the beginning of the follicular phase. By definition, the first day of menstrual bleeding is considered day 1 of the cycle. The length of the cycle is considered to be the number of days from the start of menstrual bleeding in one cycle until the start of bleeding with the next cycle. Definitions of the normal length vary. Depending on the source, the minimum normal length ranges from 21 to 24 days while the maximum ranges from 35 to 40 days. More typically, the length will consistently be close to 28 days.