THE MENSTRUAL CYCLE 



is extremel}'^ variable. Arey compiled the records of 100 girls 

 during about two years after the onset of menstruation, and 

 calculated the average cycle length of each during this epoch 

 of their lives. One-third of these girls actually never had a 

 cycle that corresponded exactly to the day with their own 

 personal averages ; in other words, every single cycle varied 

 from the arithmetical norm. During the latter part of ado- 

 lescence there is considerably greater regularity. 



The first menstruation most commonly takes place sometime 

 between the ages of 12 and 14 inclusive. The average age at 

 the time of onset, in the white race at least, is 13% years, but 

 onset at any age from 11 to 16 may be regarded as normal. 

 Delay beyond 16 is a matter for medical investigation. 



The normal duration of the menstrual flow may be from 

 one day to one week ; the modal duration is 5 days. 



Among the infrahuman primates there is only one, the 

 Rhesus monkey, which has been studied in numbers large 

 enough for positive statement. In this species, observed in 

 captivity in the United States and England, the mode, i.e. 

 the most frequent cycle length, is 28 days, as in women. 

 Individual animals have average cycle lengths of 25 to 31 

 days, and single cycles vary from 14 days up. Rather scanty 

 observations on chimpanzees, baboons, and a few species of 

 monkeys mostly show averages a little longer than 28 days. 

 This statement might or might not hold good if the statistics 

 were more extensive (Appendix II, note 10). 



The poetic suggestion quoted at the head of Chapter III, 

 that the reproductive cycles of living things are part of the 

 rhythms of the universe, must not be taken too literally. 

 Menstruation is not regulated by the moon. It happens that 

 the lunar cycle has the same length to the day as the modal 

 human cycle, but we have seen that the human cycle fre- 

 quently deviates from the mode, and if, for example, the start 

 of the period coincides with the new moon or any other given 

 lunar phase, the odds are it wiU be off cycle by at least a day 



{ 137 } 



