INTERPRETING THE TEMPERATURE READINGS

Ovulatory cycles - Identifying the temperature shift

A cycle in which ovulation has occurred is characterised by a biphasic temperature chart. The temperature remains at the lower level until the time of ovulation, when a rise or shift occurs of about 0.2 deg. C or more. The rise usually takes place abruptly between one day and the next. The temperature remains on the higher level until just before, or at the onset of, the next period.

To determine the post-ovulatory infertile phase

If pregnancy is to be avoided, intercourse cannot be resumed immediately the temperature shift is recorded. The ovum or egg can be fertilised for up to 12 hours after ovulation and allowance must be made for the possibility of a second ovulation within 24 hours of the first, a rare phenomenon which occurs in twin pregnancies.


Rule of 3 over 6

The post-ovulatory infertile phase begins after the third high temperature has been recorded.

There must be three undisturbed high temperatures above the level of the previous six daily temperatures.

The shift need only be 0.1deg.C but one of the three high temperatures should be at least 0.2 deg. C above the coverline.




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To identify the relevant temperatures when applying the rule of 3 over 6, a horizontal coverline is drawn on the line immediately above the highest of the low temperatures. A vertical line is then drawn forming a cross on the chart between the two days when the temperature shifts from the lower to the higher phase. This is illustrated above with the three higher temperatures in the upper right quadrant and the previous six in the lower left quadrant. As soon as the third high temperature has been recorded, intercourse can be resumed and the rest of the cycle will be infertile.

The 3 over 6 rule is very efficient and simple to use and for this reason is the method adopted here for identifying the post-ovulatory infertile phase for women of normal fertility.

There may however be circumstances where the use of a coverline technique may help to avoid errors of interpretation, when there is any doubt about the accuracy of the 6 low temperatures. This may occur when there is a particularly disturbed chart, or in special circumstances such as during breast-feeding, post-pill or during the pre-menopause.


Coverline technique

A horizontal coverline is drawn over all the low phase temperatures excluding temperature recordings on the first four days of the cycle, and any disturbances.

There must be a minimum of six low temperatures.

The three high temperatures must all be above the coverline.

At least one on the three high temperature should be a minimum of 0.2 deg.C

The post-ovulatory infertile phase commences after the third undisturbed high temperature has been recorded.




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As a woman gains experience in recording her temperature, she will learn to recognise her usual coverline and the normal range for her low phase temperatures and then the higher phase temperatures. The experience of past cycles can be very helpful particularly in interpreting a more difficult chart.

Variations in the temperature rise or shift

An abrupt rise is the most common with the temperature showing a sharp rise between one day and the next, although other variations may occur.

A slow rise is one in which the temperature rises slowly over several days.

A step rise is goes up in a series of steps.

These may easily be interpreted using the rule of 3 over 6.


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A saw tooth rise goes through a series of peaks and troughs and although very rare is more difficult to interpret - experienced help is needed.

By drawing a coverline, identify the beginning of the rise. The post-ovulatory infertile phase begins after the fifth temperature has been recorded.


Variations in shift day

Cycle lengths will vary considerably, but as the temperature shift occurs 12-16 days before the next period it follows that it will occur earlier in shorter cycles and later in longer cycles.

The length of the pre-ovulatory infertile phase will vary accordingly but the post-ovulatory infertile phase will remain constant.


Variations in Shift Day.
Arrows show the Temperature Shift

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A Temperature Spike

A temperature spike is defined as a single recording which is 0.2deg.C or more above its immediate neighbours.

It may be caused by a disturbance from alcohol, a late night, oversleeping, minor illness or stress. Sometimes there may be no obvious cause for a spike.

When interpreting the chart it is often helpful to circle the spike so that the disturbance is easily recognised.


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One temperature spike may safely be ignored when determining the six temperatures on the lower level, but where possible there should be an explanation for the temperature disturbance.

If more than one spike is present, then it is advisable to wait a further few days until the position becomes clear again.

If a disturbance affects one of the three higher temperatures, it is advisable to wait for a fourth high temperature to ensure infertility.


Short post-ovulatory (luteal) phase

Some women will experience cycles with a shortened post-ovulatory phase.

If the post-ovulatory phase lasts less than nine days, the cycle will be infertile as there is insufficient time for implantation to take place. This can only be seen retrospectively.

Short luteal phases may be observed during times of stress or during special circumstances (breast-feeding, post-pill or pre-menopausally)

The short luteal phase is of particular significance for couples planning pregnancy.


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Anovulatory cycles

In a small proportion of cycles, ovulation does not occur.

Anovulatory cycles are characterised by a monophasic chart, that is the temperature readings remain on one level throughout the cycle. This may be contrasted with the distinct biphasic pattern demonstrated by the ovulatory cycles.


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Anovulatory cycles are more common at the extremes of the fertile life, adolescence and the pre-menopause. They may also occur after childbirth, and after coming off the contraceptive pill.


Faulty recording technique

A very erratic temperature chart may indicate faulty recording technique.

It differs from a chart affected by illness, by showing frequent subnormal readings as well as high readings.

Erratic temperature readings are most frequently seen during the learning phase


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Temperature-taking and recording techniques need to be accurate.


Common errors include:

  • Failing to leave the thermometer in place for the required length of time

  • Alterations in recording time (with no explanation)

  • Alterations in route mid-cycle

  • Change of thermometer mid-cycle

  • Possible battery failure - digital thermometer

  • Failing to shake the mercury down properly - glass / mercury thermometer

If the temperature is being taken orally, it may be wise to change to the vaginal or rectal route at the beginning of the next cycle, if this is acceptable. This tends to give a more stable pattern which is easier to interpret.



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