Length and variation in the menstrual cycle--a cross-sectional study from a Danish county.

OBJECTIVE: To investigate the current epidemiology of menstrual patterns among women of fertile age.
DESIGN: Cross-sectional postal questionnaire study.
SETTING: County of Copenhagen, Denmark.
SUBJECTS: 3743 women, aged 15-44, selected at random from a Danish county, who were asked to provide information on menstrual pattern during the preceding year, 1981. The response rate was 78%. Information from non-responders was obtained via telephone interviews.
RESULTS: In women with regular menstrual cycles, the 5th-95th centile range of usual cycle length decreased from 23-35 days in the 15-19 years age group to 23-30 days in the 40-44 years age group. Only 0.5% of regular menstruating women had a usual cycle length of less than 21 days and 0.9% had a usual cycle length of greater than 35 days. At least one cycle length of less than 21 days was experienced by 18.6%, whereas 29.5% had at least one cycle of greater than 35 days. Menstrual cycle variation of greater than 14 days was present in 29.3% of all women. Cycle length variation of greater than 14 days was 2.7 times more frequent in women from lower social groups (controlled by age).
CONCLUSION: The study confirmed the normally used definitions of polymenorrhoea (cycle length less than 21 days) and oligomenorrhoea (cycle length between 36 and 90 days), as these very short or long menstrual cycle lengths were very seldom recorded for a longer period. However, the high frequency in a normal population of large menstrual cycle length variation challenges the view that an intra-individual variation of greater than 5 days should be regarded as a sign of disease in the woman.