In this post, we’re going to explain basal body temperature (BBT) and tell you why it’s important.
What is BBT?
When you are sleeping or very relaxed the core temperature of your body drops. This is why, for example, after getting a relaxing massage you might feel chilly. This low, resting temperature is called your basal body temperature (BBT), and it can tell you a lot about your fertility.
BBT is measured the moment you wake up, after a good night’s sleep, BEFORE you get out of bed, or do anything at all. Eating, drinking, chatting with your sweetheart, getting up to brush your teeth, etc. may raise your temperature from its resting state and obscure your BBT reading (2).
Basal body temperature can be measured orally, vaginally, or rectally. Measuring BBT orally is the standard and is just as reliable as vaginal or rectal measurements for the purposes of fertility charting. Why stick thermometers in those sensitive areas if you don’t have to? (Unless you’re into that kind of thing!)
What can BBT tell me?
Women who have natural menstrual cycles have a bi-phasic BBT pattern. Bi = two, phasic = phase, so that means your temperatures have two phases. The first phase of your cycle has low temperatures, while the second phase of your cycle has higher temperatures (3).
The first phase is called the follicular phase and starts on the first day of your period each cycle, ending at ovulation. Your BBT will show lower temperatures during this time, usually ranging from 97.0 to 97.7 degrees Fahrenheit (2).
After ovulation is the luteal phase. Progesterone, produced by the corpus luteum during your luteal phase raises your core body temperature. During your luteal phase your BBT will rise to higher temperatures, usually from 97.8 and higher (2). The range of temperatures can vary from woman to woman, as everyone is unique. But the important thing to remember is that if you are cycling naturally (not on hormonal contraceptives), your temperature chart should show this bi-phasic pattern.
BBT charting only works in women who are cycling naturally because birth control pills and other hormonal contraceptives prevent pregnancy by tricking your body into thinking it has already ovulated, so that you will not release another egg. When progesterone, which raises your temperature, is in your body consistently as is the case with hormonal contraceptives, you probably won’t have a bi-phasic pattern to chart: your temperatures will always be high.
At the end of your luteal phase your temperature will drop and you will get your period. This is the beginning of your next follicular phase. Noticing the drop in temperature at the end of your luteal phase is a great way to confirm you’ll probably get your period that day.
Why should I measure BBT?
If you are trying to get pregnant, or even if you’re not, knowledge is power when it comes to your health. BBT doesn’t tell you when you are becoming fertile — that’s what cervical mucus is for — but it may confirm ovulation (4), which no other easily-available method can. It’s easy, fast, inexpensive and accurate method that may confirm that you have ovulated (5), which urine test strips (OPKs) and ferning microscopes cannot do. And since the first thing you’ll want to know if you’re trying to conceive is if you ovulate, this is important information.
By tracking BBT, you can also determine the length of your luteal phase, which is the time between when you ovulate and when you get your period. Though the luteal phase is generally assumed to be 14 days (6), it actually can be between 12 to 16 days (2) and still be considered normal; everyone’s luteal phase is different, and you can’t safely assume that you ovulated 14 days before your period begins. If your luteal phase is too short, or you don’t produce enough progesterone, this can cause big problems when trying to conceive (2), so knowing the length of your luteal phase is important.
What’s the bottom line?
BBT is the temperature of your body at rest, unaltered by any other factors. It can be used to reliably suggest ovulation because progesterone, which is only produced during the luteal phase, raises your core body temperature. Learning about your own BBT pattern may help you identify certain fertility issues and help you get pregnant sooner (2).
(Want a highly accurate basal thermometer that's fast, fits comfortably in your mouth, and doesn't beep? Try Wink, a Bluetooth basal thermometer that syncs your temperatures to the Kindara app.)