BBT stands for basal body temperature, your body’s lowest resting temperature. It’s most commonly referenced in the context of fertility tracking, where small shifts in morning temperature can reveal whether and when you’ve ovulated. By charting these shifts over time, you can identify patterns in your menstrual cycle that help with getting pregnant or avoiding pregnancy.
How BBT Works
Your basal body temperature is the baseline reading your body settles into after several hours of sleep, before any physical activity, food, or drink. That’s why it needs to be taken first thing in the morning, ideally at the same time every day, before you even sit up in bed. Any movement, eating, or drinking can nudge the number up and make it unreliable.
During the first half of your menstrual cycle (from your period through ovulation), BBT tends to hover in a lower range. After ovulation, the ovary releases progesterone, a hormone that raises your core temperature slightly. This bump is small, typically between 0.4°F and 1°F (0.22°C to 0.56°C), but it’s consistent enough to detect with the right thermometer and a daily habit.
The key pattern is called a biphasic shift: lower temperatures before ovulation, higher temperatures after. When you see your temperature rise and stay elevated for several days in a row, that’s a strong signal that ovulation has already happened.
What You Need to Track It
A regular fever thermometer won’t cut it. You need a basal thermometer, which reads in tenths of a degree rather than rounding to the nearest whole or half degree. That precision matters because the temperature shift you’re looking for is so small. Basal thermometers are inexpensive and widely available at pharmacies.
The daily routine is straightforward but requires consistency. Take your temperature at the same time every morning, immediately after waking, before getting out of bed, eating, drinking, or talking. Record the number on a chart or in an app. Over the course of a full cycle, the pattern of lower and higher temperatures becomes visible.
What Your Chart Can Tell You
A well-kept BBT chart reveals several things about your cycle. The most obvious is confirming ovulation. If you see a sustained temperature rise lasting at least three days, ovulation almost certainly occurred just before that rise began. This is useful for timing intercourse if you’re trying to conceive, though it’s important to understand that BBT confirms ovulation after the fact. By the time you see the shift, the most fertile window has already passed.
The length of the high-temperature phase matters too. After the temperature rises, it should stay elevated for at least 11 to 12 days before dropping back down at the start of your next period. This stretch is called the luteal phase. If your temperatures consistently drop back down after fewer than 11 days, that can indicate a short luteal phase, which sometimes makes it harder for a fertilized egg to implant. It’s one of the things a doctor might investigate if you’re having trouble conceiving.
If your temperature stays elevated for 18 or more days past ovulation and your period hasn’t arrived, that’s one of the earliest possible signs of pregnancy. The sustained progesterone keeping your temperature high is the same hormone that supports early pregnancy.
Using BBT to Prevent Pregnancy
BBT tracking falls under a broader category called fertility awareness-based methods. When used perfectly and consistently, fewer than 1 to 5 out of 100 women become pregnant in the first year. With typical use, though, that number jumps to 12 to 24 out of 100. The gap between perfect and typical use is large because the method demands strict daily habits and careful interpretation, and most people slip up occasionally.
Because BBT only confirms ovulation after it happens, many people combine it with other fertility signs (like cervical mucus changes) to get a more complete picture. Relying on temperature alone leaves you guessing during the days leading up to ovulation, which are actually your most fertile.
What Can Throw Off Your Readings
Several everyday factors can make a single day’s reading unreliable. Illness or fever is the most obvious, since any infection raises body temperature independently of your cycle. Alcohol consumption the night before, emotional stress, disrupted or unusually short sleep, and changes in your waking time can all skew the number. Starting or stopping hormonal birth control also affects readings.
One off reading doesn’t ruin your chart. The goal is to see the overall pattern across the full cycle. Most people mark any known disruptions on their chart so they can mentally discount those days when looking at the bigger picture. It typically takes two to three cycles of consistent tracking before the pattern becomes clear and easy to read.