In this post, we're going to explain basal body temperature and why it’s important.
A little background:
When you are sleeping or very relaxed the core temperature of your body drops. This is why after getting a relaxing massage you might feel chilly (1). This low, resting temperature is called your basal body temperature (BBT), and it can tell you a lot about your fertility including when you likely ovulate or are pregnant (2).
BBT is measured the moment you wake up, after a good nights 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. can all raise your temperature from its resting state and obscure your BBT reading (3).
Basal body temperature can be measured orally, vaginally, or rectally, but oral BBT 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)
Why it’s important:
Whether you're trying to get pregnant or avoid pregnancy, it’s a good idea to know as much as possible about the state of your fertility. BBT doesn’t tell you when you are becoming fertile, (that’s what cervical fluid is for) what it does is confirm ovulation, which no other method easily available to you and without extensive doctor visits can. It’s easy, fast, inexpensive and accurate. Your BBT chart can confirm that you have ovulated, which urine test strips (OPKs) and ferning microscopes can not do. And since the first thing you’ll want to know if you’re trying to conceive or avoid pregnancy naturally is “Do I ovulate? If so, when?”, this is important information.
Another thing BBT can tell you is the length and health of your luteal phase. If your luteal phase is too short, or you don’t produce enough progesterone, this can cause big problems when trying to conceive. Your BBT chart will show you how long, and how healthy your luteal phase is. Again, this is very important information, not easily obtained any other way.
How it Works:
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 is has higher temperatures (4).
The first phase is called the follicular phase. This phase starts on the first day of your period, each cycle, and ends at ovulation. Your BBT will show a set of low temperatures during this time, usually ranging from 97.0 to 97.6 F or 36.11 to 36.44 C (5).
After ovulation is the luteal phase. The day after you ovulate, your BBT will rise to a higher range (usually between 97.7° and 98.3°F or 36.50° and 36.83°C) and will remain in this higher range for the rest of your cycle until your next period starts. These ranges in the follicular and luteal phases may vary from person to person as everyone is unique, but the important thing to remember is that if you are ovulating, your temperature chart will show this bi-phasic pattern (5). Progesterone, which is produced by the corpus luteum during the luteal phase, is what raises your body's temperature.
Some believe that ovulation occurs on the day of the lowest temperature before the shift (which wouldn't necessarily be the day before the shift), but this is only true in a small number of cases. Ovulation usually occurs the day before the sustained rise in temperatures (6).
At the end of the luteal phase, many observe a temperature drop that indicates their period will begin soon. The first day of your period is the beginning of your next cycle, and your next follicular phase. If conception has occurred, BBT will remain elevated for at least 18 days (5).
In summary, BBT is the temperature of your body at rest, unaltered by any other factors. It can be used to reliably confirm ovulation because progesterone, which is only produced during the luteal phase, raises your core body temperature. Learning about your own BBT pattern can help you identify fertility issues and help you get pregnant sooner.
- Weschler, Toni. (2015). Taking Charge of Your Fertility: The definitive Guide to Natural Birth Control, Pregnancy Achievement, and Reproductive Health. p62, 89, 102, 218