Basal body temperature is the body’s temperature while resting. During ovulation, BBT rises slightly. Tracking BBT over several months can help a person predict when they will ovulate and be most likely to get pregnant.

A female is most likely to get pregnant 2–3 days before ovulation, on the day it occurs, and 12–24 hours afterward.

If a person wants to conceive, they can track their BBT for a few months to understand when ovulation usually occurs within their menstrual cycle. A person can plan to have sex during their most fertile period to increase the likelihood of getting pregnant.

This article explains what BBT is and the reasons to track it. It also discusses a female’s most fertile period and how to predict it using the BBT method.

Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

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BBT is the lowest body temperature that occurs after rest. In most people, a change of 0.5–1F° will occur following ovulation.

If a person wants to use the BBT method of temperature charting, they should measure their temperature at the same time every day, first thing after waking up. It is important to take the temperature before getting out of bed, eating, drinking, having sex, or doing anything else.

There are around 6 days each month when a female can conceive. If a person is trying to get pregnant, it can be helpful to know when they are ovulating.

BBT is usually lower on the days leading up to ovulation. After ovulation, it will rise again within 24 hours and remain raised.

A person can record their daily readings on a chart to make differences easy to detect. The temperature changes are very slight, so a thermometer that measures to at least tenths of a degree is the most accurate method.

Learn more about ovulation here.

When are you most likely to get pregnant?

A person’s menstrual cycle begins on the first day of menstruation and typically lasts around 28 days, although this can vary between 21–40 days.

Ovulation occurs about 2 weeks before the start of menstruation, midway through a person’s cycle. A person is most likely to conceive on the day of ovulation, 2–3 days before, or 12–24 hours afterward. This is around days 10–16 of the menstrual cycle.

A male’s sperm can live for 3–5 days in a female’s reproductive tract. If a female has sex with a male without using a barrier method a few days before ovulation, the sperm can fertilize the egg, and they can become pregnant.

When are you least likely to get pregnant?

Once ovulation occurs and the ovary releases an egg, it has 12–24 hours for sperm to fertilize it before it dies or the body reabsorbs it. After this time, pregnancy is less likely to occur until the next menstrual cycle.

Learn more about conception here.

For most women, 97–98°F (36.1–36.6°C) is a typical BBT before ovulation. After an ovary releases an egg, BBT rises.

During ovulation, BBT will rise by 0.5–1°F on average to 97–98°F (36.1–36.6°C) and remain higher until the end of the menstrual cycle. A female’s most fertile days are 2–3 days before BBT rises.

By tracking their BBT over several months, a person can identify patterns that will help them predict when they will ovulate and optimize their chance of conceiving.

It may be challenging to interpret a BBT chart at first. A medical professional or family planning clinic can help with this.

The BBT method is not completely reliable for the following reasons:

  • BBT indicates when ovulation has already occurred, so it may not be an effective method of predicting when it will happen. For this reason, is a person should not use this method for pregnancy prevention or birth control.
  • A person must take their BBT readings at the same time every day, before getting up and before eating, drinking, or doing anything else. If they deviate from this regimen, it can affect their body temperature.
  • Other factors can affect body temperature and make it difficult to spot patterns and predict ovulation.

External factors can influence BBT readings, including:

  • drinking alcohol or smoking cigarettes the night before
  • fever
  • inadequate sleep
  • stress
  • doing anything before taking a BBT reading after waking up in the morning
  • the calibration and accuracy of the thermometer

A person should measure their body temperature first thing every morning when they wake up — before eating, drinking, having sex, or any other activity.

To measure BBT, a person can use a digital or BBT thermometer that measures in tenths of a degree.

A person can record each day’s reading on a chart. This can be on paper, an app, or online. Once a person has tracked a few months of BBT readings, they can study the pattern to identify the time in the cycle when their BBT rises.

A rise in BBT indicates that ovulation has already occurred, so a person can try to time sex on the days before the temperature rise occurs for the best chances of getting pregnant.

Taking a BBT requires a thermometer that measures to tenths or hundredths of a degree. Many are available as digital thermometers, or a person can buy a special BBT thermometer.

Below are examples of thermometers:

  • iProven: This reads to 1/100th of a degree within 60 seconds. It does not sync with an app, but the user can manually enter the information into any app or onto a paper chart.
  • Easy@Home: This measures to 1/100th of a degree in Celsius or Fahrenheit within 90 seconds. It features a large, backlit screen and stores up to 30 temperatures. Users may download an app to track temperatures but must enter them manually.
  • Femometer Vinca I: This measures to 1/100th of a degree and automatically syncs readings to an app. This thermometer features an alarm for waking up with an adjustable volume.
  • Shecare: This measures to 1/10th of a degree in 90–120 seconds and syncs with the Shecare app. This thermometer can store 400 measurements.
  • Mabis: This reads to 1/100th of a degree and comes with a 12-month supply of paper ovulation charts for tracking. It has a large display and an alarm tone when reading is complete.

BBT is the lowest temperature that occurs after rest. During ovulation, a female’s BBT rises slightly.

Tracking BBT over several months can indicate when ovulation usually occurs within a person’s menstrual cycle. If an individual wants to get pregnant, they can plan to have sex during their most fertile days.

External factors can affect BBT, such as drinking alcohol, smoking, illness, or not taking a BBT reading immediately upon waking.

To take BBT readings, a person can use a digital thermometer that measures to tenths or hundredths of a degree.