Basal body temperature, more commonly referred to by its shorter acronym BBT, is a tool for assessing fertility, More specifically, it is used to confirm that ovulation is happening, when it happened (past tense is important here), and also provides information on other aspects of health including giving insights on hormone levels (like progesterone) and providing a window into thyroid health.
Simply, BBT tracking tracks your lowest (or near to) resting temperature. We take BBT immediately upon waking, before moving around or doing anything else that might raise the temperature. BBT can be taken, most commonly, via one of three main methods: oral, auxiliary (underarm), or vaginal. Most women opt for oral temping. However, for those wishing to use BBT to track thyroid health, auxiliary temping is generally recommended.
So how does BBT tell us if we have ovulated? Firstly, know that BBT can only indicate ovulation *retrospectively*. It can never be used to “predict” ovulation (that doesn’t exist). Because before ovulation, when estrogen is the dominant hormone, BBT is lower. But once ovulation has occurred, the corpus luteum (what was formerly the follicle which house the maturing ovum) begins to release progesterone. And progesterone causes body temperature to rise! So when you take you BBT every day, you’ll notice that (if you ovulated) your BBT will be low for a while, and then will either suddenly or gradually rise and your temps will remain higher than your pre-ovulatory temperatures until your next menstruation.
So when you see that up-shift in your temperature, when marked on a graph and in the absence of any causes of disturbance like fever, you’ll be able to know retrospectively that you ovulated!
BBT is the “thermal” part of Symptothermal methods of fertility awareness — the Fertility Awareness Methods which track (at least) two main signs of fertility: cervical mucus/fluid (CM or CF) which is the primary sign of fertility, and basal body temperature. Simplistically, cervical mucus indicates when the window of fertility opens, indicating rising estrogen, and BBT confirms when ovulation has occurred and signals the end of the window of fertility.
Cervical mucus (which we can talk about more to endless detail!) is produced by crypts inside the cervix under the influence of estrogen and progesterone — each hormone causing a different type of cervical mucus to be produced. Learning to know the difference between these types is one of the most essential parts to practicing the Symptothermal and cervical mucus-only methods with high efficacy.
Learning to correctly collect this amazing first-hand data on your body day-by-day, and to accurately record and interpret it according to a specific system — and then to make choices about your sexual activity based upon that accurate collection, recording, and interpretation — is the only way that you will be able gain the high rate of effectiveness that Fertility Awareness is loved for.
Because if you’re not using one consistent (and science-backed) method of tracking this information, and making the autonomous choice to daily follow the rules of one chosen method, **you are not using Fertility Awareness** but are instead winging it! This is so important, because I see a lot of women who don’t invest the time and care into learning one specific method, and instead claim to “just know their body really well” or pick and choose what they (inconsistently) track — and they mistakenly think that they are practicing Fertility Awareness.
I can promise you — will never regret taking the time to actually learn how to track your cycles. By learning from an educator you’ll be able to have full confidence in your comprehension of your method, and you’ll know that you’ll be able to practice FAM with a high efficacy rate (so long as YOU follow the rules, because it is a user-dependant — aka empowering — method). In fact all of the high effectiveness rates that you see are from studies on women who have received training from qualified Fertility Awareness Educators!