Fertility Education Trust

guidelines to charting your fertility cycle, page 23 of 26

4. The Double-Check Concept: The Calendar / Mucus Double-Check Rule (before ovulation)

If you have diary records of your cycle lengths over the last year:

Many women record their periods in their diaries. This is a great help when starting to learn NFP. (However, if someone has been using the Pill or IUD, these are not natural cycles and cannot be counted.) Work out the length of each cycle precisely. A cycle begins on Day 1 of period and ends the day before the next bleed starts. Put the cycle lengths in a line:

Take the shortest cycle over the last year - in this example the shortest cycle is 27 days.
Subtract 20 days (i.e. 14 days back to ovulation, 6 days for sperm survival in mucus) = Day 7.

Day 7 is therefore the last infertile day before ovulation based on calculation, that has been infertile in the all cycles of this particular woman over the past year. (In the short 27 day cycle, she could have conceived on Day 8). Statistically, Day 7 should continue to be infertile for the next year, provided no mucus is been seen or felt prior to that day.

In this example, if the couple really do not want to become pregnant, the rule recommends that they do not continue intercourse beyond Day 7 for the next year, in case that short cycle is repeated. After Day 7, it is best wait to resume after ovulation, in the absolutely infertile time.

After a year of charting, if that short cycle re-appeared, then Day 7 continues to be the last statistically infertile day, double checked by DRYNESS.

On the other hand, if the cycles have lengthened and the shortest cycle is now for example 28 days, then using the same formula: “shortest cycle minus 20 = last infertile day, if still dry”, Day 8 would now become the last infertile day - if still DRY.

In each case, mucus observations are key to success. A short cycle can occur at any time for some women, totally out of the blue, and mucus observation will detect it, if the woman is vigilant.