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Reviewed by the Faculty of Harvard Medical School

Rhythm Method

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.

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 thru 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.


  • Perfect use of this method results in a 2 percent to 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.


  • 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.

Last updated October 1, 2010