Helping You Monitor Your Fertility

Recommended Pages for Further Reading


"Unlike other fertility awareness techniques, the rhythm method can be rather inaccurate. Other symptoms such as BBT and CM should also be tracked to more accurately predict fertility."


Ovulation Calendar


The rhythm method is a very simplistic approach to estimating fertility. It is can offer a rough estimate of when ovulation will occur. This technique is not always accurate. It is therefore important not to put too much emphasis on the results. Ovulation can be much better estimated by charting daily symptoms like basal body temperatures and cervical mucus.

Limitations of the Rhythm Method

The rhythm method tries to predict ovulation by looking for a rhythmic pattern in cycles. Many generalizations are needed for this to work. In the end it may not work for all women. Even when it does work for an individual, it may not work consistently from cycle to cycle. This technique is only recommended if you are content with a very rough estimate of ovulation. It is important not to place too much faith in it due to the shortcomings of the technique. Couples interested in more accurately tracking fertility should record other symptoms using the fertility awareness method as described on this site.

There are two assumptions made by the rhythm method. It tends to work well only if both are true.

  1. The luteal phase is fixed at 14 days.
  2. The female cycle is roughly the same length every cycle.

The first assumption is generally upheld. The luteal phase does not vary much from cycle to cycle for a given individual. It can vary from person to person though. Even if the luteal phase is shorter or longer than 14 days, the calculations can be modified to use a different value. The difficulty is knowing what value to use. If fertility symptoms are recorded and charted, a woman's actual luteal phase length can be calculated. Without this information though, most users must assume the default length of 14 days in their calculation.

The second assumption is more problematic. This technique will not work at all for women with irregular cycles. The predicted ovulation date is only valid if each menstrual cycle is the same length. The calculation assumes that the current cycle will be of average length. If it is not, the calculated ovulation date will be wrong. The actual length of a cycle is not known until it is over though, so it is difficult to anticipate when this technique is going to work.

How it works

In the menstrual cycle the luteal phase tends to be of fixed length. Any variation in the length of the cycle is primarily due to variation in the follicular phase. This information is used to estimate ovulation.

It would be easy to calculate ovulation if a woman knew exactly when her next menstrual cycle would begin. Since the luteal phase is fixed, all that is required is to count backwards from the start of the next cycle towards the ovulation date. Ovulation is easily determined.

The dilemma is that women do not know when their next cycle will start. This is especially true if they have irregular cycles. The rhythm method approximates the start of the next cycle using information about cycle history. It basically assumes that your current cycle will be of average length and be followed by the start of your next cycle. If this is true, the method works well. Frequently it is not though and the calculated ovulation is off.

Not all cycles may be of average length. The rhythm method tries to account for this by building in uncertainty around the ovulation date. This is often kept smaller than it should be since the technique tends to self destruct under these circumstances. Instead of identifying an ovulation date, it may identify 12 day window where ovulation might occur.


  1. Take the length of your shortest cycle you have had and subtract 19. This method assumes that you will be infertile up until this cycle day.
  2. Take the length of your longest cycle you have had and subtract 10. This yields the beginning of the assumed infertile period after ovulation.
  • Shortest Cycle: 29 days
  • Longest Cycle: 32 days
Days 1 through 10 are assumed to be infertile (29-19=10). Fertility is assumed from day 11 through 21 until cycle day 22 (32-10=22). From cycle day 22 onward, the infertile period is entered and the chance of pregnancy is assumed to be low.

Recording on Charts

The rhythm method is generally not recorded on charts. Some charts may identify ovulation based upon this approximation, but no daily symptoms are specified.


  • Requires minimal effort. You do not need to record daily symptoms
  • Can offer an initial rough estimate of ovulation until other methods are able to identify ovulation.


  • Not always accurate
  • Does not work for women with irregular cycles
  • Creates a false sense of confidence of when ovulation occurs