A recently developed variant of the Rhythm Method is known as the Standard Days Method. Developed by Georgetown University's Institute for Reproductive Health, the Standard Days Method has a simpler rule set and is more effective than Rhythm. A product, called CycleBeads, was developed along side the method to help a user keep track of high and low fertility points during a menstrual cycle. The Rhythm Method is sometimes considered a form of fertility awareness. However, modern techniques of fertility awareness generally rely on several physiological signs of fertility, rather than a simple counting of days. Because of its lower accuracy, many fertility awareness teachers consider calendar rhythm to have been obsolete for at least 20 years.

At one time, the Rhythm Method was sanctioned as an acceptable form of natural family planning by the Catholic Church, although, as in the fertility awareness community, more modern methods are being advocated by the church today.

Description and effectiveness
Most menstrual cycles have several days at the beginning that are infertile (pre-ovulatory infertility), a period of fertility, and then several days just before the next menstruation that are infertile (post-ovulatory infertility). The first day of red bleeding is considered day one of the menstrual cycle. The formula for Calendar Rhythm requires the woman to know the length of her menstrual cycles.

To find the length of the pre-ovulatory infertile phase, nineteen (19) is subtracted from the length of the woman's shortest cycle. To find the start of the post-ovulatory infertile phase, ten (10) is subtracted from the length of the woman's longest cycle.

A woman whose menstrual cycles ranged in length from 30 to 36 days would, according to the Rhythm Method, be infertile the first 11 days of her cycle (30-19=11), be fertile on days 12-25, and resume infertility on day 26 (36-10=26). When used to avoid pregnancy, the Rhythm Method has a perfect-use failure rate of 9% per year.

The Standard Days Method may only be used by women whose cycles are always between 26 and 32 days in length. In this system, days 1-7 of a woman's menstrual cycle are considered infertile. Days 8-19 are considered fertile. Infertility resumes beginning on day 20. When used to avoid pregnancy, the Standard Days Method has a perfect-use failure rate of 5% per year.

Imperfect use of the Rhythm Method would consist of not correctly tracking the length of the woman's cycles, thus using the wrong numbers in the formula, or of having intercourse on an identified fertile day. Imperfect use is fairly common, and the actual failure rate of the Rhythm Method is 25% per year.

Reasons for high failure rate
The Rhythm Method formula makes several assumptions that are not always true.

The postovulatory (luteal) phase has a normal length of 12 to 16 days, and the Rhythm Method formula assumes all women have luteal phase lengths within this range. However, many women have shorter luteal phases, and a few have longer luteal phases. For these women, the Rhythm Method formula incorrectly identifies a few fertile days as being in the infertile period.

The Rhythm Method uses records of past menstrual cycles to predict the length of future cycles. However, the length of the pre-ovulatory phase can vary significantly, depending on the woman's typical cycle length, stress factors, medication, illness, menopause, breastfeeding, and whether she is just coming off hormonal contraception. If a woman with previously regular cycles has a delayed ovulation due to one of these factors, she will still be fertile when the Rhythm Method tells her she is in the post-ovulatory infertile phase. If she has an unusually early ovulation, the Rhythm Method will indicate she is still in the pre-ovulatory infertile phase when she has actually become fertile.

Finally, the Rhythm Method assumes that all bleeding is true menstruation. However, mid-cycle or annovulatory bleeding can be caused by a number of factors. Incorrectly identifying bleeding as menstruation will cause the Rhythm Method's calculations to be incorrect.