THE HORMONES IN HUMAN REPRODUCTION 



as statisticians say) in adult European and American women 

 is 28 days, but it is also very common for a woman to average 

 cycles of 25, 26, 27, 29, and 30 days. The individual woman, 

 moreover, often varies several days, in any one cycle, from 

 her own average. To state this in exact terms, so as to make 

 clear just how much variation a woman may consider normal, 

 is rather difficult, for it is a matter of statistics and such 

 things are hard to translate into everyday language. The 

 clearest statement is that of Professor Arey, already quoted, 

 whose words I paraphrase as follows: let a woman keep a 

 record of her cycles for several years, so that she has enough 

 observations to strike an average. Say, for example, that her 

 personal average is 28 days. Arey's figures show that with 

 this information she cannot hope to predict the onset of any 

 given period with accuracy closer than 2.5 days plus or minus, 

 i.e. she may expect it any day between the 25th and 30th 

 after onset of the last period, and even then one-third of all 

 her cycles will depart still more widely from the average 

 length, say another day or two, and sometimes more. The 

 statement that she averages a 28-day cycle has the same kind 

 of meaning as the average price of eggs during the year, 

 which may be very different from the price at any one time, 

 or a baseball player's average of home runs per game. All it 

 means is that the length of her successive cycles will be within 

 a few days, more or less, of 28 days, seldom however coming 

 out at that precise length. In fact a woman whose cycles were 

 perfectly regular to the day, during many months or years, 

 would be a medical curiosity. No such case has ever been 

 reported. 



When we come to consider, a little later, the intricate inter- 

 play of hormones that goes to produce the cycles, we shall 

 not be surprised that the timing is not perfectly regular. Nor 

 will we be surprised to learn that in young girls, during the 

 first few months or years of menstrual function, while the 

 endocrine mechanisms are becoming adjusted, the cycle length 



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