Basal body temperature (BBT) is your body’s temperature when you’re fully at rest. It’s the lowest natural temperature your body reaches during a 24-hour period, typically measured first thing in the morning before you get out of bed, eat, or drink anything. Most people track BBT to identify ovulation patterns, either to conceive or to avoid pregnancy, because this resting temperature shifts in a predictable way across the menstrual cycle.
Why Your Temperature Changes With Ovulation
After you ovulate, your ovaries release progesterone to prepare the uterine lining for a potential pregnancy. Progesterone also has a heat-generating effect on the body, raising your resting temperature by about 0.5 to 1°F (roughly 0.3 to 0.6°C). This increase is small enough that you won’t feel it, but a thermometer picks it up clearly.
Before ovulation, during the first half of your cycle, BBT typically sits in a lower range. After ovulation, it shifts upward and stays elevated until your period arrives. If you chart these temperatures over several cycles, you’ll see a clear two-phase pattern: lower temperatures, then higher temperatures. The point where the shift happens marks ovulation.
The catch is that BBT only confirms ovulation after it’s already occurred. Your most fertile days are the two to three days before the temperature rise, so BBT charting works best for learning your body’s patterns over time rather than predicting fertility in real time.
How to Measure It Accurately
Take your temperature every morning immediately after waking up, before any activity. That means before getting out of bed, before reaching for water, before checking your phone and sitting up. Even small movements generate body heat that can throw off the reading. You need at least a few hours of unbroken sleep beforehand for the measurement to be reliable.
You’ll want a basal thermometer rather than a standard fever thermometer. A regular digital thermometer works in a pinch, but basal thermometers display temperature to the tenth of a degree, which matters when you’re tracking shifts as small as half a degree. You can measure orally or vaginally, but pick one method and stick with it throughout the cycle, since the two sites give slightly different readings.
Record the temperature on a chart or app every day. Consistency in timing matters too. Taking your temperature at 6 a.m. one day and 9 a.m. the next introduces variation that can obscure the real pattern.
Reading Your Chart: The Thermal Shift
The goal of charting is to spot the moment your temperatures shift from a lower pre-ovulation range to a higher post-ovulation range. One common method uses a “coverline,” a horizontal reference line drawn just above your highest pre-ovulation temperatures. To identify the end of the fertile window, you look for three consecutive temperatures recorded at least 0.15°F above the coverline. Once you see that sustained rise, ovulation has passed.
Not every chart looks textbook-perfect. Some cycles show a gradual climb rather than a sharp jump. Others have a single high temperature caused by a poor night’s sleep or a glass of wine, which can look like a false shift. This is why tracking over multiple cycles gives you a much clearer picture than relying on a single month.
What Can Throw Off Your Readings
Several everyday factors can cause your BBT to spike or dip independently of your cycle. Alcohol consumption the night before tends to raise morning temperature. So does illness, even a mild cold. Disrupted or insufficient sleep, shift work, jet lag, and sleeping in a significantly warmer or cooler room all introduce noise into your data. Some medications can also affect readings.
When you know a reading is unreliable, note it on your chart but don’t use it to draw conclusions. Most charting apps let you flag questionable temperatures so they don’t skew the overall pattern.
BBT as a Family Planning Tool
Fertility awareness-based methods, which include BBT tracking, have a wide range of effectiveness depending on how consistently they’re used. With perfect use, meaning you follow the method correctly every cycle, fewer than 1 to 5 out of 100 women become pregnant in the first year. With typical use, that number jumps to 12 to 24 out of 100. The gap reflects how easy it is to make mistakes: missing a morning reading, misinterpreting a chart, or not abstaining during the fertile window.
BBT is more effective when combined with other fertility signs, like changes in cervical mucus, rather than used alone. Because the temperature shift only confirms ovulation after the fact, relying on BBT by itself leaves you guessing about the days leading up to ovulation, which are your most fertile.
BBT and Early Pregnancy
After ovulation, your temperature normally stays elevated for about 10 to 14 days, then drops as progesterone falls and your period begins. If your temperature remains high for 18 or more consecutive days past the thermal shift, that sustained elevation can be an early indicator of pregnancy. The continued progesterone production needed to maintain a pregnancy keeps your temperature up.
This isn’t a substitute for a pregnancy test, but for someone who’s been charting, it’s often the first clue that something is different about a given cycle.
BBT and Thyroid Health
You may have heard that a consistently low body temperature signals an underactive thyroid. There is a real connection: thyroid hormones regulate metabolism and heat production, and cold intolerance is a well-known symptom of hypothyroidism. A large study found a weak statistical association between hypothyroidism and lower body temperature.
However, the relationship isn’t strong enough to be diagnostically useful. Normal resting body temperature varies widely in the general population, with 95% of healthy people falling between 96.3°F and 99.1°F (35.7°C and 37.3°C). If you used the old textbook cutoff of 98.6°F (37°C) as a dividing line, more than three-quarters of the healthy population would appear hypothyroid. The British Thyroid Foundation has noted that using body temperature to diagnose thyroid problems is statistically less reliable than flipping a coin. Blood tests remain the only dependable way to assess thyroid function.
That said, if your BBT charts consistently show unusually low temperatures and you’re also experiencing fatigue, weight changes, or difficulty with your menstrual cycle, those patterns together are worth mentioning to your doctor. The temperature alone doesn’t tell the story, but it can be one piece of a larger picture.