What Is Basal Body Temperature?

Basal body temperature, commonly abbreviated as BBT, is the lowest temperature your body reaches during rest. It is measured first thing in the morning, before you get out of bed, eat, drink, or engage in any physical activity. This resting temperature provides a reliable baseline because it is not yet influenced by the metabolic demands of your daily routine.

For most women, BBT during the follicular phase of the cycle, the time between menstruation and ovulation, typically ranges from 97.0 to 97.7 degrees Fahrenheit (36.1 to 36.5 degrees Celsius). After ovulation, the hormone progesterone causes body temperature to rise by approximately 0.2 to 0.5 degrees Fahrenheit (0.1 to 0.3 degrees Celsius), where it remains elevated throughout the luteal phase until the next period begins, or continues to stay elevated if conception has occurred.

Tracking BBT is one of the oldest and most accessible fertility awareness methods. It requires only a basal thermometer, which measures temperature to one-hundredth of a degree, and consistent daily recording. While BBT cannot predict ovulation in advance the way ovulation predictor kits can, it confirms that ovulation has taken place and provides valuable data for understanding your unique cycle pattern over time.

How Temperature Shifts Indicate Ovulation

The temperature shift that follows ovulation is driven by progesterone, which is produced by the corpus luteum, the structure that remains in the ovary after the egg is released. Progesterone acts on the hypothalamus, the brain region that regulates body temperature, causing a measurable increase in BBT. This rise typically occurs within one to two days of ovulation and persists for the remainder of the cycle.

To confirm ovulation using BBT data, you look for a sustained temperature increase of at least 0.2 degrees Fahrenheit that lasts for three or more consecutive days compared to the previous six readings. This pattern is often referred to as the "three over six" rule and is a widely accepted standard in fertility awareness practice. The day before the temperature shift is considered the most likely day of ovulation. Because the shift only confirms ovulation after it has already happened, BBT charting is most useful when combined with forward-looking methods such as cervical mucus monitoring or ovulation predictor kits to time intercourse during the fertile window.

Best Practices for BBT Tracking

Accurate BBT tracking depends on consistency. Take your temperature at the same time every morning, ideally after at least three consecutive hours of sleep, and before standing up, talking, or reaching for your phone. Even small variations in routine can affect the reading. Place the thermometer on your bedside table so it is within easy reach, and record the reading immediately to avoid forgetting.

Use a dedicated basal thermometer rather than a standard fever thermometer, as basal thermometers measure to two decimal places and detect the subtle shifts that matter for fertility tracking. You can take your temperature orally or vaginally, but choose one method and stick with it throughout the cycle, as the two sites can yield slightly different readings.

Note any factors that might influence your reading, such as illness, poor sleep, alcohol consumption the night before, or taking your temperature at a different time. These disrupted readings can be marked on your chart so they do not confuse the overall pattern. Over time, charting will reveal your personal biphasic pattern, meaning the clear two-phase temperature graph with lower follicular temperatures and higher luteal temperatures. Most women need two to three cycles of consistent tracking before the pattern becomes clear and reliable for fertility planning.

Understanding the Biphasic Pattern

A biphasic temperature chart is the hallmark of an ovulatory cycle. The term "biphasic" simply means "two phases" and refers to the distinct lower-temperature follicular phase followed by the higher-temperature luteal phase. When you plot your daily BBT readings on a chart, an ovulatory cycle will show a visible step up from the first phase to the second, creating a pattern that resembles a staircase or a plateau.

If your chart does not show a clear biphasic pattern after several cycles of consistent tracking, it may indicate that ovulation is not occurring regularly. Anovulatory cycles, where no egg is released, tend to produce flat or erratic temperature charts without a sustained rise. This information is clinically valuable and worth sharing with your healthcare provider. A monophasic chart, combined with other signs such as irregular periods or difficulty conceiving, can prompt further investigation into hormonal health. Conversely, a consistently strong biphasic pattern with a luteal phase of 12 to 16 days is a reassuring sign of healthy ovulatory function.