The Rhythm Method for Birth Control

The first formal calendar-based method of birth control was developed in 1930 by John Smulders, a Roman Catholic physician. This system was based on knowledge of the menstrual cycle and for several decades was the primary form of birth control available to Catholic couples who didn't want to become pregnant. In 1999, there was a new development in the basic calendar-based birth control method known as the Standard Days Method which promoted the use of a product known as CycleBeads intended to help the woman keep track of fertile days. While some people believe the fertility awareness is essentially the same as the rhythm method, fertility awareness generally involves tracking cervical mucus, cycle length and body temperature with a goal of becoming pregnant. By determining the length of prior menstrual cycles, the rhythm method can be used to time unprotected intercourse for days identified as infertile (if the goal is to avoid pregnancy) or for days identified as fertile should the couple want to become pregnant.

It was a Dutch gynecologist who discovered that once per menstrual cycle is how often women ovulate. Later, in the 1920's, a Japanese gynecologist studied the subject and determined ovulation generally occurs some two weeks prior to the woman's next menstrual period. These discoveries were used to develop a system which could be used by infertile women to be able to best time their intercourse days to enable them to become pregnant. John Smulders took this research and used it to create the rhythm method, used primarily to avoid pregnancy. Most women's menstrual cycles include a few days at the beginning which are infertile, then a period of fertility, and finally several days before the next menstrual period that are infertile. The first day of bleeding is considered day one of the menstrual cycle, however for the rhythm method to be successful, a woman must know the length of her menstrual cycles-not knowing fairly accurately the length of cycle could result in incorrect tracking, thus having unprotected intercourse on a fertile day. In order for the rhythm method to be highly successful, the woman must have the discipline to keep accurate records and abstain from unprotected intercourse during fertile periods. The failure rate of the calendar-based rhythm method is 25% over the course of a year.

How to Calculate

There are two basic methods of calculating fertile days. The first method subtracts nineteen from the woman's shortest menstrual cycle. The resulting figure is the estimated length of the pre-ovulatory infertile phase. To find the estimated starting point of the infertile phase prior to ovulation, ten is subtracted from the woman's menstrual cycle which lasted the longest. Therefore if the cycle ranged from 30-36 days in length you would be likely to be infertile for the first 11 days of your menstrual cycle (30-19 = 11); days 12-25 would be considered fertile, while day 26 would be considered infertile. When the rhythm method is the only form of birth control used in a perfect manner, the failure rate is estimated to be 9% per year. If you are looking for a method that is somewhat simpler, take a look at The Standard Days Method. If your menstrual cycle is consistently between 26 and 32 days, then to calculate more simply, days 1-7 would be considered infertile, days 8-19 would be considered fertile and from day 20, you would be once again considered infertile. When used in a perfect manner, the Standard Days Method has a failure rate of 5% per year.

Although this method of birth control lacks many of the health concerns of other contraceptives, it does have a higher failure rate. It is assumed the fairly high failure rate of the rhythm method is due to the formulas which are not always true for every woman. The postovulatory phase has a normal length of 12-16 days, and the rhythm method is calculated on this "normal" length, however many women have shorter postovulatory phases while others have longer ones. For those women, the rhythm method would incorrectly identify a few fertile days as being infertile.

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