Every cycle is different, and in order to ensure the reliability of the symptothermal method, certain rules must be followed. This may seem complicated at first, which is why the Ovy App automatically applies the rules and makes charting easier. The basic principle is always double-checking: basal temperature must be combined with the observation of cervical mucus.

Rules for determining the infertile phase after ovulation

Temperature: Three consecutive measurements must all be higher than the highest of the six preceding measurements (= reference line).
Cervical mucus: After the peak day, i.e. the last day with the best mucus quality (usually S+), at least three days with poorer mucus quality must follow.
Cervix: The cervix must have been closed and firm for at least three days.
Infertile phase: The infertile phase begins on the first evening when two of the above rules and the temperature rule are fulfilled.

Rules for determining the infertile phase before ovulation

Temperature: Only if a clear temperature shift was identified in the previous cycle can an infertile phase be assumed at the beginning of the next cycle.
5-day rule: For beginners, the first five days of the cycle may be considered infertile, provided that a temperature shift (first higher measurement, FHM) occurred in the previous cycle. If the first higher temperature occurs on cycle day 12 or earlier, the Minus-8 rule applies from then on.
Minus-8 rule: After twelve documented cycles, the last infertile day at the beginning of the cycle is defined as the earliest first higher temperature measurement minus eight days.
Cervical mucus: Both of the previous rules only apply as long as no cervical mucus is observed or no sensation of wetness is felt.
Cervix: Both of the previous rules only apply as long as no changes in the cervix can be detected after menstruation.

Exception and special rules

In rare cases, the above rules are supplemented to ensure accurate identification of the fertile phase. The Ovy App applies all exception and special rules automatically:

Exceptions for temperature

• Exception 1: If the third higher temperature measurement is less than 0.2°C above the highest of the previous six values, a fourth measurement must be awaited. This fourth value must be above the reference line but no longer necessarily 0.2°C higher.

• Exception 2: During the three higher measurements, one value may be at or below the reference line. This value is then disregarded. In this case, four measurements in total are required. The two exceptions cannot be combined.

Special rules for cervical mucus observation

Special rule: If within the three days after the presumed mucus peak (usually S or S+) the same quality as the peak occurs again, the count starts over until three days with poorer quality confirm the infertile time.

Special rule: If, after mucus evaluation is completed but before temperature evaluation is finished, cervical mucus of the same quality as the peak appears again, mucus evaluation must be repeated (three days with poorer mucus after the “new” peak).

Exception for the infertile phase before ovulation

If a menstrual calendar has been kept for twelve months, beginners may calculate the infertile phase at the beginning of the cycle differently: shortest documented cycle length – 20 = last infertile day at the beginning of the cycle.

Which factors can distort basal temperature?

Not every body reacts the same way, which is why there are no universally valid disturbance factors that always cause a temperature rise. To find out what counts as a disturbance for your body, you should carefully record changes in your daily life from the very beginning. This way, you can exclude disturbed values to avoid distorting the evaluation.

Common disturbance factors that often cause temperature changes are:

• Drug use

• Unusual alcohol consumption

• Short or disturbed sleep

• Going to bed later than usual

• Night sweats or chills

• Late evening meals

• Staying out late/partying

• Stress & excitement

• Travel & environmental changes

• Time change

• Colds or other illnesses

The Ovy App evaluates your data according to the rules above. You simply need to measure your temperature, enter it into the app, and document your cervical mucus.

You might also be interested in these articles