Leslie B. Arey

The Degree of Normal Menstrual Irregularity: An Analysis of 20,000 Calendar Records From 1,500 Individuals

Data from 12 different studies have been assembled covering actually recorded cycles for 1500 women. 8462 cycles (585 women) were 28 days long. Average length of all cycles is 33.9 days for girls and 2 8.4 days for women. The commonest length of 17652 cycles (the grand mode of the individual modes of 1265 women) is between 30 and 31 days for pubertal girls and 27 days for adult women. Mean length (based on individual averages) is 33.6 days for girls and 29.5 days for women. These means and modes however have little significance in pubertal girls because of wide variability encountered. Maximun departure from individual mean varies from 1 to 69 days in women and 6 to 211 days in pubertal girls. The smooth frequenecy distribution obtained in group studies is the result of overlapping individual irregularities and not of closely spaced regular individual cycles. Cycles of 3 5 6 and 7 weeks do not appear to exist in sharp contrast to conclusions based upon oral testimony. Cycle length is extremely variable in pubertal girls; in middle adolescence (Cycles 25-39) regularity improved so that 2/3 were within a 10-day range; at the end of adolescence 2/3 were within plus or minus -4.4 days; and after age 21 about 2/3 were within plus or minus -2.5 days. However among adults cycles ranging from 2 to 3 weeks to 7 or more weeks appear in all groups for which data were collected. In records of more than 500 women 27% never showed their own means during the observation period which averaged 11 cycles in length. These studies reported in detail came from 11 different investigators and represented all ages many nationalities and various grades of society. Irregularity not perfect regularity is the common situation.