Temperature (12) | Natural Family Planning Teachers Association of Ireland (NFPTAI)

A brief outline of ‘why basal body temperature is used as a fertility indicator’ is given below on this page.

Click on any of the following headings listed below for further information on basal body temperature.

What does the Temperature Chart look like? (12a)

Planning Pregnancy – Is Temperature any help? (12e)

Why is Basal Body Temperature (BBT) used as a fertility indicator?

Both Dr W. Squire, London in 1868, and Dr Mary Putnam Jacobi, Philadelphia independently in 1878 noted that the early morning temperature, (basal body temperature, see below), in women of child-bearing age was higher during part of the menstrual cycle but they did not know the reason for this. In 1904, Van de Velde in Holland noted that the rise in temperature (‘thermal shift’) is due to ovulation, (release of an egg), and in 1928 he pointed out that the rise in BBT is due to progesterone secretion from the corpus luteum in the ovary, (see below).

The rise in basal body temperature (BBT), called the ‘thermal shift’ is the sign of ovulation. When the woman takes and records her temperature every day during the cycle and if she has ovulated in that cycle, the record shows that the temperature is low in the first part of the cycle before ovulation, and high in the second part of the cycle after ovulation. This temperature pattern showing a phase when the temperature is low followed by a phase when the temperature is higher is called a biphasic temperature pattern and is the pattern typically seen in the menstrual cycle when ovulation has occurred.

Does the thermal shift confirm that ovulation has occurred?: The ‘thermal shift’ or rise in basal body temperature is the only fertility indicator that confirms ovulation (release of an egg). This means that the ‘thermal shift’ is the most useful fertility indicator to confirm the end of the fertile phase.

How does the ‘thermal shift’ indicate the end of the fertile phase?: The ovum (egg) lives for just 24 hours after ovulation, therefore 24 hours after ovulation the ovum is dead and no further ovulation will occur in that cycle. For this reason the phase of the cycle after ovulation is called the definitely infertile phase.

When does the woman know that the definitely infertile phase of the cycle has begun?: When the woman observes a sustained rise in temperature as defined by the Rules of the ‘symptothermal double-check method’ of natural family planning she knows that the definitely infertile phase of the cycle has begun.

WHAT CAUSES THE RISE IN TEMPERATURE AFTER OVULATION ? After the ovum (egg) is released from the ovary (i.e. ovulation), the cells lining the ruptured ovarian follicle remaining in the ovary become a small gland called the corpus luteum which secretes the hormone progesterone. The rise in basal body temperature is due to progesterone and therefore the ‘thermal shift’ as defined by the NFP Rules indicates ovulation has occurred as progesterone is present only after ovulation.

WHAT IS THE BASAL BODY TEMPERATURE (BBT) ?: The basal body temperature is the temperature taken immediately on waking, before getting out of bed, and after a minimum of 3 hours continuous sleep. No food or drink should be taken before the temperature is taken. The temperature must be taken at the same time every morning as normal body temperature varies during the day because of the circadian rhythm. The temperature difference between the lower-phase temperature and the higher-phase temperature may be quite small and as taking the temperature an hour later or earlier than usual will give a higher or a lower reading, this emphasises the importance of recording the temperature at the same time every day.

DEFINITION OF A THERMAL SHIFT: A thermal shift is defined by the Rules of the ‘Symptothermal Double-Check Method’ of natural family planning and these Rules must be taught to the woman by a qualified NFP teacher.

To be most effective, the woman must be taught the symptothermal double-check method of Natural Family Planning by a qualified natural family planning teacher.