The three stages are called early, middle and late transition. The time from age 35 until the first change begins is called pretransition. The postmenopausal phase is after the final menstrual period, a phase which is determined by 12 months elapsing with no bleeding or spotting. Middle transition occurs when menstrual cycles during midlife become irregular but do not involve skipping a period. An irregular cycle is when there are more than six days difference between two consecutive cycle lengths.
Late transition occurs in midlife when skipping periods begins.
In order to understand the menopausal transition it is crucial to study the whole spectrum of menstrual cycle changes, such as flow and cycle length changes, rather than just irregularity. It is also critical to begin studying these changes earlier than age 45, the age at which many studies enroll women. Women were asked about the first persistent changes they had noticed in their menstrual cycles during midlife. These stages are determined from the information provided by the yearly health update questionnaire and by the menstrual calendar.