Women who want to keep track of when they ovulate often choose to use a basal thermometer. These thermometers may operate the same way as a regular thermometer, but their accuracy and purpose are different. Because a basal thermometer is usually more expensive than a regular thermometer, it is typically only purchased as a fertility aid, not for everyday use.
A basal thermometer is designed for reading the small changes in temperature that occur over the course of a woman's menstrual cycle. A woman uses a basal thermometer to record her temperature every morning just after waking up and charts the numbers to determine the best day to attempt to conceive a child. A regular thermometer is used to check the overall body temperature to detect or monitor a fever.
A basal thermometer and regular thermometer can be digital or mercury-based, although the majority of thermometers sold today are now digital. A mercury thermometer contains the element mercury in a thin glass tube. The mercury rises and falls when there is an increase or decrease in temperature. A digital thermometer uses a small computer to read the temperature and displays the number in a small window on the side of the thermometer. Digital thermometers read the temperature more rapidly than mercury thermometers, often within 30 to 60 seconds.
A basal thermometer is intended for oral, vaginal or anal use, although most woman use them orally. Regular thermometers can be designed for use orally, anally or under the arm. Some digital regular thermometers can be embedded into an infant pacifier, a strip to be placed on the forehead or a device to point into the ear. Basal thermometers are used first thing in the morning each day, while a regular thermometer can be used whenever a fever is suspected.
Basal body thermometers are more accurate than regular thermometers, but they tend to operate in a smaller range. While a regular thermometer is accurate to 0.2 degrees F, a basal thermometer is accurate to 0.1 F. Because ovulation prediction depends on noticing very slight changes, usually between 0.4 to 1.0 degrees over the course of a month, the increased accuracy of a basal thermometer allows more accurate charting than a regular thermometer.
While a regular thermometer is typically not accurate enough to chart ovulation, it is perfectly fine to use a basal thermometer to detect a fever. A basal thermometer is usually used in conjunction with other methods of detecting ovulation, such as cervical mucus observation.
- BabyCenter: Basal Body Temperature and Cervical Mucus
- "Taking Charge of Your Fertility"; Toni Weschler, MPH; 2001