Whether for religious reasons or a desire for a natural form of birth control, many couples use the rhythm method. If they are trying to avoid pregnancy, they will abstain from sexual intercourse during a woman’s fertile time. Before a couple can effectively use this method, however, they must understand the woman’s cycle in order to determine when she typically ovulates. This can be done with a calendar and some simple calculations.
Temporarily Use Other Birth Control
You have to use the calendar method for several months before you can establish when your fertile days are. If you are using the calendar method as a birth control, use an alternate form of birth control during these months.
Charting Your Cycles
On a calendar, mark the first day of your cycle, which is the first day of your period. On the first day of your next period, count the days since the previous time you started your period to the day before you just started. For instance, if the first day of one period was on February 17 and the next period starts on March 16, you would count from February 17 to March 15–a 27-day cycle. Write this number of days on the calendar for the last month. Repeat this for at least six months.
Finding the First Fertile Day
Look over the months and determine what your shortest month was; subtract 18 from that number. For instance, if your shortest cycle in the past six months was 25 days, you would calculate 25 — 18 = 15. Begin with the first day of your last period, count ahead 15 days and mark it. This will typically be the beginning of your fertile time.
Finding the Last Fertile Day
Look over the months and find your longest cycle; subtract 11 from that number. For instance, if your longest cycle was 32 days, you would calculate 32 — 11 = 22. Begin with the first day of your last period, count ahead 22 days and mark it. This will typically be the end of your fertile time.
According to the Mayo Clinic, for women who have regular cycles the calendar method has an 87-percent effectiveness rate. For women with irregular cycles, this is not a good method to use to avoid pregnancy.
Pros and Cons
Pros: There are no side effects from using the calendar method to determine fertility. It does not cost anything to use this method. Cons: You have to chart your cycle for at least six months before you can establish your typical fertile days. It doesn’t protect against sexually transmitted diseases.