Fertility Education Trust

guidelines to charting your fertility cycle, page 21 of 26

4. The Double-Check Concept

Dr. Anna Flynn FRCOG, who set up the NFP Teachers Association, dedicated much of her time as a gynaecologist to teaching Natural Family Planning, to researching and refining the rules to make it as efficient as possible. She soon realised that the more indicators a woman / couple could use to double-check their symptoms, the higher the success rate.

the infertile phase after ovulation

There were already two indicators to define this phase:

the mucus changes after PEAK DAY
the rise in temperature after ovulation

When both were used to double-check each other, the couples were very confident and the success rate was high. When the post ovulation phase alone is used, the pregnancy rate is virtually zero.

the infertile phase before ovulation

There was only one indicator - the onset of mucus - to mark the start of the fertile time. DRY days, with no mucus seen or felt, are in theory infertile, but only once a woman has learned how to recognize her symptoms accurately.
When women had to rely entirely on that one indicator, particularly in the learning phase, it was too easy to make a mistake, to misread the signs and so encroach on the fertile phase.
When couples used the pre- and post- ovulation phases, the surprise pregnancy rate was higher, showing that the difficulty lay with the pre-ovulation phase.