CALENDAR CALCULATION

In using the sympto-thermal method, a calculation based on past cycle lengths can be used to back up the mucus symptom as a double-check in identifying the last day of the pre-ovulatory infertile phase.

The calculation is derived from part of the old calendar or rhythm formula.

The original calendar method also includes a calculation to define the end of the fertile phase, but if using the sympto-thermal method, this is unnecessary as the double-check of temperature and mucus symptom is more reliable.

Calender or Rhythm Method

The Calendar method is based on the knowledge that ovulation occurs 12-16 days before the following menstruation regardless of the overall length of the cycle.

From a record of at least six cycles the lengths of the shortest cycle and the longest cycle are noted.

Allowing three days for the viability of the sperm in the female genital tract and two days for the life of the ovum and taking into account that ovulation might occur on any of 5 days in each cycle a formula was evolved:

Shortest cycle (S) minus 20
= Last infertile day of the pre-ovulatory phase
Longest cycle (L) minus 10
= Last fertile day
For example:-
Length of cycles during last six months = 28, 29, 28, 27, 30, 28
(S = 27)
S - 20
= Last infertile day
27 - 20 = 7
(L = 30)
L - 10
= Last fertile day
30 - 10 = 20


The last infertile day according to the calculation is day 7 and the last fertile day is day 20.

To avoid conception, couples were therefore recommended to abstain from intercourse, from the 8th-20th days inclusive.

Calendar or rhythm method gives an unnecessarily long time of abstinence.

It is now known that the ovum can be fertilised only within a few hours of ovulation and the viability of the sperm in the genital tract is variable and may be longer than three days.

Although the calendar method is not sufficiently reliable to be recommended for use as a single indicator, the information gained by recording the length of cycles and their variability is useful.




Detecting the early infertile phase by a calculation

Use of the calculation Shortest cycle minus 20 is most valuable for couples requiring a high degree of effectiveness, but wishing to have intercourse in the pre-ovulatory relatively infertile phase.

During the learning phase many women find difficulty distinguishing the early mucus changes.

The use of this calculation acts as an additional safeguard to warn of the onset of the fertile phase.


Shortest cycle minus 20 calculation
Estimate the shortest cycle length over the previous year,
e.g. Cycle lengths - 32, 29, 31, 30, 30, 28, 31, 29, 30, 31, 30, 29,
Shortest cycle (S) is 28. Subtract 20 to give the last infertile day
S minus 20 = Last infertile day
28 - 20 = 8. Last infertile day is day 8


For many women the S minus 20 calculation will determine the end of the pre-ovulatory infertile phase a day or two before the warning given by first mucus symptom.

However if mucus is noted before the day designated by the calculation, then this is significant and would indicate the onset of the fertile phase.

The S minus 20 calculation should be continuously reassessed to monitor the shortest cycle over the previous year by recording the current and the shortest cycle length at the top of each chart.
When a woman has recorded at least six cycles (preferably 12) including temperature recordings, she can add a slightly more accurate estimate of the onset of the fertile phase based on the earliest shift in temperature. The 'Doering Rule' is discussed in the book 'Fertility'


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