The Degree of Variability of the Length of the Menstrual Cycle in Correlation with Age of Woman

This study examined the calendar records of 2149 menstrual cycles experienced by 592 health Swiss women in 1935-55 in order to assess the correlation between age and length of menstrual cycle. Qualitative changes in the menstrual cycle were observed in the adolescent maturity and climateric phases with adolescent women demostrating a descreasing range of variability and mean length of menstrual cycles and women in the climateric phase manifesting an increasing range of variability and average length of menstrual cycles. Overall 80% of the menstrual cycles analyzed fell into the 24-34 day range with an average cycle length of 29.2 days. Between ages 11-20 years the average length of the menstrual cycle decreased from 30-9 to 28.5 days and the standard deviation decreased parallel with the mean length of the cycle. From 21-28 years of age the average length of the cycle increased slightly and the standard deviation progressivey decreased indicating a rapid decrease in the frequency of long cycles. From 29-42 years of age the average cycle length decreased arallel with the continuous decrease of the standard deviation. Finally 43 years of age marked the onset of a simultaneous increase of the average length of the menstrual cycle and the standard deviation.