A basal thermometer is an ultra-sensitive thermometer used to take the basal (base) body temperature, the lowest normal body temperature. This temperature typically occurs during sleep, so the basal temperature is normally measured immediately upon waking. It has been suggested that List of temperature sensors be merged into this article or section. ... One of the many changes that take place in a womans body during her menstrual cycle is an increase in body temperature at the onset of ovulation. ...

Whereas ordinary household thermometers express body temperature to only one decimal place (e.g. 98.6 degrees Fahrenheit), basal thermometers express body temperature to two decimals places (e.g. 98.56 degrees Fahrenheit).

Basal thermometers are useful for charting a woman's fertility cycle, since basal body temperature typically rises when ovulation occurs. This information can be used to help maximize or minimize the chance of pregnancy. Fertility Awareness (FA) is the practice of observing one or more of a woman’s primary fertility signs to determine the fertile and infertile phases of her menstrual cycle. ... Ovulation is the process in the menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum (also known as an oocyte, female gamete, or casually, an egg) that participates in reproduction. ...

Basal thermometers are used as part of the Sympto-Thermal method of Natural family planning. Natural Family Planning (NFP) is a set of Catholic-sanctioned methods of family planning, which help women to achieve or avoid pregnancy by identifying times of infertility and potential fertility. ...