The Calendar Rule is a statistical estimate based on three facts:

  1. The post-ovulation phase (luteal phase) has a constant length which is on average 14 days (range 10 to 16 days), and it tends to be the same length for any individual woman for each cycle.1
  2. The viability of the sperm is five days in fertile-type mucus in the pre-ovulation phase.2
  3. Knowing the client’s record of the cycle lengths of each of her preceding six or more menstrual cycles.

The calendar rule cannot be used in the first six cycles, as there is insufficient data.
The calendar rule is a minor fertility indicator and is never used alone but is used in combination with cervical mucus observations to define the beginning of the fertile phase of the cycle.3 

The client must be taught the application of the calendar rule, and the rules defining the beginning and end of the fertile phase by a qualified NFP teacher.

References:

  1. Klaus H; ‘Natural family planning – Is it scientific? Is it effective?’ page 7; Newman Lecture Series 1-May 21, 2000. (online).
  2. Wilcox AJ, Weinberg CR, Baird DD; ‘Timing of sexual intercourse in relation to ovulation – effects on the probability of conception, survival of the pregnancy, and sex of the baby’; N Engl J Med 1995; 333;1517-21
  3. Flynn AM, Brooks M; The Manual of Natural Family Planning, 1996, ISBN 0 7225 3115 X
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