The duration of the menstrual cycle in four-weekly or three-weekly cycles. With these records, it is the easiest way to obtain data concerning menstruation and pregnancy. It was only when experiments with healthy women were made that the regularity of the menstrual cycle clearly emerged.
It was found that; 90% of women as a rule have a regular monthly cycle lasting twenty-eight days. 97% of cases the duration of the menstrual cycle were always regular. 22% of these women menstruated punctually every twenty eight days while 39% every thirty days, and 11% every thirty-two days. That makes a total of 72% proportion of women in whom menstruation was most regular. The remaining menstruated at shorter or longer intervals, but these intervals recurred quite regularly. Only in 3% of the selected healthy women was there a considerable irregularity.
It is a characteristic fact that the great majority of women whom themselves are unaware how long their menstrual cycle lasts. Many women, if asked, say they have a regular twenty-eight day cycle, but if they are observed for several months it is found that the cycle lasts either twenty-seven or twenty-nine days, or even alternately twenty-seven, twenty-eight or thirty days. Some experience shorter cycle of twenty to twenty five days.
First draw a simple table with vertical column and indicate January to December, and draw horizontally 30 columns and indicate 1-30 as days. The duration of the actual menstruation is of no importance. Mark the date of commencement of the following menstruation in the particular column and do so for the next month. Now, count the days that have elapsed between the commencement of the first and second menstruations (including the first day of the first period, but not the first day of the second menstruation).
If the result is the same for a whole year, in such a case, a woman has a regular menstrual cycle and therefore a so-called "simple menstrual cycle," that is to say, one which always lasts exactly the same number of days. A "simple menstrual cycle" may also last twenty-seven, twenty-eight, twenty-nine, thirty-one days, etc.; the important thing is that it should always last an equal number of days.
But if the duration of the menstrual cycle is, for instance, one month twenty-seven days and the next thirty days, it is no longer a simple, but a so-called "double menstrual cycle."
Finally, it may happen that a woman has on one occasion a twenty-seven-day cycle, on the second occasion a twenty-eight day cycle, and on the third occasion a thirty-day cycle. This is called a "triple menstrual cycle."
Take for example a woman that has a simple menstrual cycle of thirty days. She knows that, for instance, her menstruation begins on May 1st, it will end on the 30th of that month. Now, if she counts fifteen days backwards from May 30th, she will know that her ovulation takes place on May 16th. She is also able to tell in advance that her next menstrual cycle will end on June 29th, and if she again deducts fifteen days (always including the last day of the menstrual cycle) she can establish in advance that her ovulation during that cycle will take place on June 15th. Thus women with simple menstrual cycles are able to calculate their ovulation dates far ahead. A calendar made up in this manner is what we call an Ovulation Calendar.
The matter is somewhat more complicated in the case of women with double or multiple menstrual cycles. Here is a practical example: This woman first menstruated since beginning the calendar on March 1st, and her next menstruation began on the 28th of that month, so that on this occasion she had a twenty-seven day cycle. Deducting fifteen days, her ovulation must have taken place on March 13th. The third menstruation began on April 29th, so that this cycle lasted thirty-two days, and going back for fifteen days, we find that ovulation must have occurred on April 14th. The fourth menstruation began on May 29th, i.e., thirty days after the previous menstruation on April 29th, so that this time her cycle was thirty days and, deducting fifteen days from the last day of her cycle, i.e., from May 28th, her ovulation during this cycle must have occurred on May 14th.
Now, this woman, according to her menstrual calendar, sometimes had a twenty-seven-day cycle, and sometimes a thirty or thirty-two-day cycle. In this case it is not easy to fix the day of ovulation, as the woman cannot know in advance whether her current menstrual cycle will last twenty-seven, thirty or thirty-two days. She only knows that she ovulates fifteen days earlier than the last day of her cycle, but she cannot determine the date of that fifteenth day.
It will be seen that a woman with a double or multiple menstrual cycle can never determine the exact day of her ovulation. However, there is no reason for her to despair. She knows that the maximum duration of her cycle is, say, thirty-two days, and also that the minimum duration is twenty-seven days, having kept her menstrual calendar for a whole year. If she wishes to calculate the date of her next ovulation, she will reason it out as follows: My last menstruation began on May 29th.
If my next cycle is of the minimum duration, i.e., twenty-seven days, then my next menstruation must begin on June 25th, in which case the ovulation would fall on June 10th. If, however, the next cycle happens to be of the maximum duration, i.e., thirty-two days, then my next menstruation will fall on June 30th, and ovulation on June 15th. And if the cycle is one of thirty days, my next menstruation will fall on June 28th and ovulation on the 13th. I am able to establish from this that whatever the duration of my next cycle is going to be, ovulation cannot take place either before June 10th or after the 15th, therefore it falls between those two dates.
It will be seen from the above that a woman with a simple menstrual cycle can calculate the exact date of ovulation, while a woman with a double or multiple menstrual cycle can only calculate the exact period of days during which ovulation must take place, and she can do this with all the greater care, the shorter the difference is between the maximum and minimum duration of her cycle.