The oldest known(indeed many people still believe that this is the only one) is the . This is also the least reliable of the s. As its name implies it is based on the dates of the woman’s menstrual cycle; a record needs to be kept for twelve months before you begin to use this method, to ascertain the pattern of your menstrual cycles.
On average, women ovulate 14 days before the beginning of each period; in practice it is more likely to be within two days either way, i.e. 16-12 days before a period. Since sperm can live for up to four days, then four days before the earliest possible time of ovulation are also unsafe — this gives a date of 20 days before a period. As an ovum can survive for up to forty-eight hours, then two days after the latest possible time of ovulation are also unsafe — this gives a date of 10 days before a period. If you have a perfectly regular 28day cycle, days 9-19 inclusive (this is important) are unsafe. If you have a regular 25-day cycle, days 6-16 inclusive are unsafe. For a 32-day cycle, days 13-23 are unsafe.
Most women’s menstrual cycles are not the same length each month, which is why a twelve month record needs to be kept to make this method at all accurate. For instance, you may find that your shortest cycle is 25 days and your longest 30. Because it is impossible to predict at the beginning of a month whether this is going to be a long cycle or a short one, you need to build in extra precautions to cover either eventuality. To find your earliest possible unsafe day you will need to take 20 days from your shortest ever cycle: this gives day 5. To find your latest possible unsafe day, you need to take 10 days from your longest ever cycle: this gives day 20. Therefore from day 5 to day 20 inclusive of every monthly cycle you will need to abstain from sex. That’s over two weeks of every month! Thecombines the maximum amount of abstention for the minimum amount of reliability; if you want to use a of birth control, you would do much better with one of the others.