Post-menarcheal menstrual cycle in British (Newcastle upon Tyne) girls.

Data for the first 36 menstrual intervals immediately following menarche were analysed for 298 girls studied in Newcastle upon Tyne. The mean cycle length decreased from 49 . 5 days for the first cycle in 29.5 days for cycles 31-36. There was no clear seasonal pattern in cycle length. The standard deviation within subjects in the length of menstrual interval fell sharply from 23 . 3 days for cycles 1-6 to 10-7 days for cycles 7-12 and more slowly thereafter, reaching 5 . 5 days for cycles 31-36, and was not related to age at menarche. The proportion of short cycles (17 days or less) was relatively stable, but that of long cycles (57 days or more) decreased with cycle number from 27.4% for the first cycle to about 1% for cycles 31-36. The average length of menstrual flow was 5 . 5 days irrespective of age at menarche, cycle number of season. The standard deviation of menstrual flow within subjects fell from 1 . 5 days for cycles 1-6 to 0 . 8 days for cycles 25-36.