The menstrual cycle is a series of regular, natural changes in the body. The length of a menstrual cycle is determined by the number of days from the first day of one period to the first day of the next. In other words, the first day of the menstrual cycle is the first day of your period.
The menstrual cycle has two main phases. The follicular phase, or proliferative phase, normally makes up the first 10–16 days of the cycle. The luteal, or secretory, phase comes right after ovulation and lasts for about 14 days, ending with menstruation if pregnancy didn’t occur. Generally, menstrual cycles that last from 21–35 days, with a typical duration of 28 days, are considered normal.