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.
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.
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.
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.