Body temperature is a vital marker for the reproductive system. Medical researchers discovered that the body temperature is elevated at the luteal phase of the menstrual cycle due to the corpus luteum formation from the ovarian follicle and subsequent activation of corpus luteum and the follicle arereleasing ova. This discovery helps in the advancement of modern reproductive treatment and also natural family planning. Clinically, basal body temperature (BBT) monitoring is a usual method for ovulation detection. The simple and least invasive technique of the basal body temperature monitoring system is convenient to check the ovulation status. The release of progesterone hormone gives thermogenic effect and increases BBT.
The menstrual cycle has four phases such as menstruation, follicular, ovulation, and luteal phases. The BBT alters depending upon these phases. The range of BBT during the follicular phase is in between 97.0 and 98.0°F, which is considered as a lower range of BBT. This temperature is maintained until one day before ovulation. The BBT reaches its lowest point during ovulation. After ovulation, the BBT raises 0.5–1.0°F and this temperature maintain throughout the luteal phase due to progesterone releases from the corpus luteum. But corpus luteum becomes degenerated at the advanced stage of the luteal phase and consequently serum progesterone level decreases. The outcome of this leads to a lowering of BBT and it is noticed 1 to 2 days prior to menstrual bleeding starts.
The fertility window of a woman can be checked through regular monitoring and maintaining the BBT chart by measuring oral, rectal and vaginal temperature after wake up in the morning and before starting any activity. A modern digital thermometer or conventional glass thermometer can measure the BBT and specialized BBT charts, smart-phone apps or online services are available to record the temperature. Experts suggested BBT measure is not suitable for women who are seeking for pregnancy however; it is a better approach for natural contraception or ovulation recording.
“Cover line” is one of the interpretation methods of BBT. However, BBT interpretation is not an easy task, as emotional or physical stress, alteration of room temperature, fever, antipyretics medications, alcohol, sleep disturbance, change of waking time, climatic change, and commencement or discontinuation of contraceptive pills can influence BBT. It is also necessary to mention that the prevalence of increased infertility may due to stress-induced hypothermia, as excessive stress can alter body temperature by inducing sympathetic nerve dominance and resultant of which leads to a disturbed menstrual cycle.
In 2005, a “GAP” technique was demonstrated by the experts and considered as a better approach than the ‘cover line’ method. In “GAP” technique, cohabiting male partner’s BBT is subtracted from the female partner’s BBT and the difference is recorded as “gap”. Theoretically, this difference is only influenced by female hormonal changes but no environmental factors influence this gap. But the limitation of this approach is women who have not to live with their male partners, they unable to use this technique.