Objective: To characterize premenopausal menstrual regularity and the patterns of divergence from regularity associated with the approach of the final menstrual period.
Design: Two samples of individual cycle length sequences contributed by participants in a population-based longitudinal study of the menopausal transition were examined. The first sample, of “early” sequences, is used to characterize menstrual regularity. The second shows how cycle length patterns change as the final menstrual period (FMP) is approached. Regression slopes are used to measure trend in cycle length, and changes in cycle length variability are registered by a simply calculated measure, the “running range.”
Results: Sequences in the early cycles sample rarely varied outside the 21–35 day range and did not show a rising or falling trend. In contrast, pre-FMP sequences generally became increasingly variable in length, while rising above 35 days in mean during the last 10 cycles. The variability measure remained below 40 days throughout the early sequences, but characteristically rose above 42 days during sequences including the last 20 pre-FMP cycles. In early sequences, but not in pre-FMP sequences, long and short cycles tended to alternate.
Conclusions: Increased variability is the dominant feature of cycle length pattern for most women as their final menstrual period approaches. Underlying this is a steady trend toward mean cycle lengths above 35 days. An indicator of the approach of menopause is a rise in running range of cycle lengths to 42 days.