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Rhythm Method – Fertility Awareness

The rhythm method, also known as “fertility awareness,” is a method of birth control that uses the menstrual cycle to predict your most fertile time of the month – that is when you are most likely to become pregnant. Once you have identified your most fertile time you simply don’t have sex or use a barrier method of birth control during that time period.

Fertility awareness birth control methods are based on the fact that:

  • sperm may live in the female reproductive tract for up to 7 days;
  • the female egg lives for only a day;
  • fertilization may occur even days after intercourse;
  • “safe days” occur 2 days after ovulation and continue until your next period

The calendar rhythm method helps you predict your most fertile days each month by knowing your average menstrual cycle length.
Menstrual cycle length is the average number of days between the first day of a menstrual cycle and the first day of the next menstrual cycle. Menstrual cycle length normally varies by a day or two each month. The average length of the menstrual cycle is 28 days, plus or minus two days. That said, normal menstrual cycles can be as short as 24 days or as long as 32 days. But ovulation (release of an egg) almost always happens 14 days before the first day of menstrual bleeding.

You need to know how to predict what day of your cycle you are most likely to ovulate. Knowing your fertile time can help with planning or preventing pregnancy. Your fertile time is when the egg is released and can be fertilized. Predicting the fertile time is based on a 28-day menstrual cycle:

Day 1 of your menstrual cycle is the first day of your menstrual blood flow. Your fertile days are most likely from day 10 through day 17 of the cycle. If your average cycle length is different from the standard 28-day cycle then the fertile time must be adjusted to the length of your average cycle. For instance, if your cycles range between 26 to 30 days, then your fertile time would be from day 8 through day 19.

Rhythm Method – Fertility Awareness

To determine this apply the following formula:

  • Subtract two days from day 10 of the 28-day cycle: 10-2=8
  • Add two days to day 17 of the 28-day cycle: 17+2=19

For example, if your cycles range between 25 and 31 days, then:

  • Subtract three days from day 10 of the 28-day cycle: 10-3=7
  • Add three days to day 17 of the 28 day cycle : 17+3=20

Your fertile time will be days 7 – 20 of your menstrual cycle. Remember, day 1 of your cycle is the first day of your menstrual flow. By marking the fertile days of your cycle on your calendar each month, you can know in advance when to avoid intercourse.

Another approach involves a slightly different calculation:

  • Subtract 18 days from your shortest cycle length, and subtract 11 days from your longest cycle length.

The result will define your fertile time. For example, if your shortest cycle is 22 days (22-18=4) and your longest cycle is 27 days (27-11=16) then your fertile time will be cycle days 4 to 16. You will need to avoid intercourse for these 12 days if you want to avoid pregnancy.

Because menstrual cycle lengths vary from person to person, and often from month to month in the same woman, you cannot assume your fertile days will be exactly the same days every month. By keeping track of your menstrual cycle for at least eight months before relying on this method, you will be more certain about your average cycle length and how much it varies.

By doing this, you will be more confident of when you are most fertile and be able to avoid intercourse. Also be aware that the length of your “typical” cycle may vary with your age as you grow older.

Rhythm Method Advantages

  • Perfect use of this method results in 2-3 percent pregnancy rate per year;
  • This method does not cost anything, and there is no need for medications or surgery;
  • This method does not interrupt or alter fertility. Pregnancy is possible as soon as this method is abandoned.

Rhythm Method Disadvantages

  • When used imperfectly, this method has a pregnancy rate as high as 84 percent;
  • Because of normal variations in the menstrual cycle and in the lifespan of eggs (one to three days) and sperm (two to seven days), you will need to abstain from intercourse for a number of days, including those immediately around ovulation, to be safe;
  • This method is not appropriate for women with irregular menstrual periods. Even for women with regular menstruation, the calendar method is less dependable than symptom and temperature monitoring or cervical-mucus monitoring.

(info from


Rhythm method (Knaus-Ogino method)

To find the estimated length of the pre-ovulatory infertile phase, nineteen (19) is subtracted from the length of the woman’s shortest cycle. To find the estimated 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 be estimated to be infertile for the first 11 days of her cycle (30-19=11), to be fertile on days 12-25, and to resume infertility on day 26 (36-10=26). When used to avoid pregnancy, the rhythm method has a perfect-use failure rate of up to 9% per year.


A software program developed in Germany from 1995 to 2001, Perimon is a stricter variant of the rhythm method. It requires a greater period where unprotected sex is not allowed when used to avoid pregnancy, designating a maximum of 10.5 days each cycle as infertile. No clinical studies have been done to determine effectiveness, but the program’s developers claim a perfect-use failure rate of 4% per year. The Perimon software requires a paid subscription.

Standard Days Rhythm Method

Developed by Georgetown University Institute for Reproductive Health, the Standard Days Method has a simpler rule set and is more effective than the rhythm method. A product, called CycleBeads, was developed alongside the method to help the user keep track of estimated high and low fertility points during her menstrual cycle. 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 is considered to resume beginning on day 20. When used to avoid pregnancy, the Standard Days Method has a perfect-use failure rate of 5% per year. According to, women in their late 30s and early 40s tend to have shorter menstrual cycles. Consequently, the fertile phase may begin on day five rather than day eight.

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