Women in early transition are those with changes in flow amount, flow duration and/or cycle length that begin sometime in midlife. 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. A skipped period occurs when twice the modal (most commonly reported) cycle length elapses. Thus, if periods are 60 days apart (two periods) and the modal cycle length that year is 27 days, it is a skipped period. If it is less than 54 days (twice 27), then this is a delayed period.

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. Of all the initial menstrual cycle changes reported by the women in this study, 37% occurred before age 40 and an additional 41% occurred between the ages of 40 and 44. By age 45, 78% of the women in this study had already experienced their first menstrual cycle changes.

Women were asked about the first persistent changes they had noticed in their menstrual cycles during midlife. Of those who responded

--34% said their periods came closer together (menstrual cycle lengths were shorter)

--32% said the flow during their periods was lighter

--29% said the flow during their periods was heavier

--24% said their periods lasted fewer days

--20% said their periods lasted more days

--14% said their periods came more irregularly (menstrual cycles were more irregular compared to the cycles before age 35).

These stages are determined from the information provided by the yearly health update questionnaire and by the menstrual calendar.