Data on 8,308 menstrual cycles from 1,740 South Indian women prospectively recorded were analyzed to identify the effect of age on menstrual cycle length. The distribution was skewed to the right with the mean (SD) cycle length of 31.8 (6.7) days. The range of 25-40 days constituted 10-82 per cent of menstrual cycle lengths. In no age group did 28-day cycles occur in more than 9 per cent of women. Variability as measured by the standard deviation was high among those below 19 years of age, stabilized during 25-39 years, and then increased in women aged 40 years or more. The findings are discussed in the light of other studies and possible implications in fertility control programs.
PIP: Between 1969 and 1974, a community survey consisted of following 1740 currently married women living in urban and rural areas in North Arcot district in Tamil Nadu, India, for 8308 cycles to examine the association between age and menstrual cycle length. Interviewers went to the homes of the women every 5 weeks. Mean cycle length by age of women was 31.2 days, with a standard deviation (SD) of 3.2 days. 39% of the women had a mean cycle length of more than 32 days. 60% had mean cycle lengths ranging from 25-31 days. Mean menstrual cycle length among the 8308 cycle lengths was 31.8 days with a SD of 6.7 days. This mean cycle length was considerably different from that of earlier studies. The most frequent cycle length was 30 days. 43% of cycle lengths were more than 31 days. 47% were between 25-31 days. 28-day cycles occurred in no more than 9% of the women regardless of age. Variation of cycle lengths was greatest following menarche (SD - 5.6 days for women = or 19 years old). The variability fell during the main reproductive years (SD = 3.2 days for 20-24 year olds and 2.8 days for 25-29 year olds) and began to climb as women approached menopause (SD = 2.9 days for 30-34 year olds and 4.8 days for women = or 35 years old). These findings indicated that menstrual cycle lengths of between 25 and 40 days were normal. The researchers believed that the 28-day cycle is not the norm, particularly in developing countries. They suggested that clinicians should reconsider the decision-making process currently used when determining menstrual cycle abnormality based on a 28-day-cycle norm. These findings are especially important in regards to women applying the rhythm or natural family planning methods.