Basal body temperature, or BBT, is the lowest body temperature that a person has on any given day. Throughout your cycle, your BBT will change based on fluctuations in your hormone levels, specifically the hormone progesterone.
How is BBT relevant to fertility?
Changes in basal body temperature can signal an upcoming or past ovulation. Typically, BBT will dip to the lowest point right before you ovulate, and spike to the highest temperature in the two to three days following ovulation, with a big spike about 12 to 24 hours after ovulation.
This spike is caused by the sudden increase of progesterone in your system. After ovulation, progesterone is released by the matured ovarian follicle (now known as the corpus luteum) that had held the egg being released that month during your period. This hormone plays a big role in preparing the uterus for pregnancy, and in maintaining pregnancy after conception. It also increases your body temperature, which will stay high throughout pregnancy if that egg gets fertilized, or lower normal if the egg is not fertilized.
While this spike in BBT won’t be helpful in letting you know that you’re still ovulating, it can let you know that you’ve just ovulated, which is helpful information in figuring out the pattern of your menstrual cycle. On the other hand, the dip in BBT can let you know that ovulation is just around the corner. Tracking can help you get a better sense of your fertile window from month to month.
How do I track it?
Since BBT is the lowest temperature your body reaches in the normal course of the day, to get the most accurate results, it’s best taken first thing in the morning, right when you wake up – before you even get out of bed! – before you are active, and your body starts heating up. BBT should be measured with an oral thermometer and, it can be helpful to keep an oral thermometer at your bedside and make a habit of taking your temperature as soon as you wake up – turn off your alarm, take that temp, and track your info in Ovia!