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Basal Body Temperature (BBT)

by Dr. Tessa Commers
Julie Medical Advisor
General Sex

Understanding Basal Body Temperature: A Comprehensive Guide

For anyone who’s chosen the natural birth control route (aka, not using hormonal birth control or intrauterine devices), tracking body basal temperature (BBT) can help you determine when you ovulate so you can avoid sex or use a temporary form of protection when your chance of becoming pregnant is at its highest. BBT plays a unique role in reproductive health. Although some people swear by its use to prevent pregnancy, it’s not perfect and requires a lot of personal research ahead of time. Find out more about what your basal body temperature is, how it works, and how to measure it accurately to help you avoid pregnancy.

How does it work

First, it’s important to know that BBT and regular body temperature are not the same thing. Body temperature typically fluctuates throughout the day due to various factors, like exercise, eating, and your environment. On the other hand, BBT is your body's lowest resting temperature and is best measured in the morning before you even get out of bed.

When your BBT rises, that’s a sign that ovulation (when an egg is released from your ovary) is happening or has just happened. The temperature increase stems from the increased progesterone, a hormone responsible for preparing your uterus for pregnancy. Though it’s not an exact prediction of the moment you’re ovulating, tracking it over time can give you a sense of when it’s likely to occur. If you’re trying to prevent pregnancy, it can help you understand when to avoid sex (or take extra precautions) during your cycle.

Keep in mind: once you ovulate, the egg will survive in your fallopian tube for up to 24 hours before dissolving. But sperm can survive in your reproductive system for up to 5 days. If you have sex even a few days before ovulation, you have a higher chance of becoming pregnant. Learn more about conception here.

How to take basal body temperature

Start recording your BBT on the first day of your period. Continue to take it every morning throughout your next few cycles. It may take several cycles to notice a consistent pattern, so be patient.

Here are some other steps to keep in mind:

  • Use the right thermometer: Make sure you have a thermometer that measures temperatures to at least one-tenth (or two decimal places) of a degree. This will help you detect subtle temperature changes.

  • Take your temperature when you wake up: Keep your thermometer within easy reach next to your bed. When you wake up in the morning, take your temperature before you do anything else, even before you even sit up or reach for your phone.

  • Consistency is key: Aim to take your BBT at the same time each morning. Waking up at different times can affect the results.

  • Measure from the same place: Take your temperature from the same place on your body every day. Most measure in the mouth (like taking your regular body temp), but some prefer vaginal or rectal measurements.

  • Record your temperature daily: Maintain a record of your daily temperatures using a graph, spreadsheet or fertility-tracking app. This will help you identify patterns and trends over time.

  • Identify a temperature increase: BBT typically rises less than half of a degree Fahrenheit about halfway through your cycle. This increase is a key sign that ovulation is occurring or just did.

  • Determine your fertile period: You’re most likely to get pregnant in the 5 days leading up to and 1 day after ovulation. Trying to conceive? This is your window to get it on. Trying to prevent pregnancy? This is your window to not get it on or to use a backup form of birth control, like condoms.

Why should I use the basal body temperature method?

The BBT method offers a natural and inexpensive way to track your menstrual cycle, fertility, and ovulation. It’s a great option for anyone looking to avoid the cost of hormonal birth control and the side effects that can come with it. However, becoming pregnant is the biggest side effect if the method fails.

What is a normal basal body temperature?

Everyone’s BBT is different but it only varies by a degree or two. Here’s the average estimate:

Before ovulation: The typical BBT range before ovulation falls between 96 to 98 degrees Fahrenheit (35.5 to 36.6 degrees Celsius). This represents your body's lowest resting temperature.

After ovulation: BBT typically ranges from 97 to 99 degrees Fahrenheit (36.1 to 37.2 degrees Celsius). The degree of increase can vary, with some individuals experiencing as little as a 0.4-degree Fahrenheit (0.22-degree Celsius) rise, while others may see an increase as high as 1 degree Fahrenheit (0.56 degrees Celsius).

How do I know when I’m ovulating using basal body temperature?

You won’t know you’re ovulating until you’re in the middle of it or it has passed (although your body can give you clues as to when you’re ovulating, called ovulation symptoms.)  That’s why you should track your BBT through several menstrual cycles to understand when your BBT could rise. When your BBT has stayed at a higher temp for three consecutive days, that’s when you’ve most likely ovulated.

Basal body temperature while pregnant

If you become pregnant, your BBT should remain elevated above your pre-ovulation baseline, and you won’t see the drop that normally happens before a period is about to occur.

There isn't a specific "normal" range for BBT during pregnancy, just what’s normal for you.

Factors that affect your basal body temperature

BBT isn’t the most reliable method to prevent pregnancy because many factors, even common lifestyle ones, can influence it. Here are the things that are most likely to affect your BBT reading:

  • Breastfeeding: Breastfeeding can affect hormonal levels, potentially leading to fluctuations in BBT.
  • Drinking alcohol: Alcohol consumption can disrupt sleep patterns and, consequently, influence BBT readings.
  • Fever caused by illness or infection: Elevated body temperature due to fever can temporarily raise your BBT.
  • Not getting enough sleep: Sleep disturbances can affect the consistency of BBT readings.
  • Stress: High stress levels can lead to hormonal fluctuations and influence your reading.
  • Taking some medications: Certain medications can affect hormonal balance and BBT readings.
  • Traveling to different time zones: Travel-induced sleep disruptions and changes in your daily routine can influence BBT.

Use Julie as a backup for the BBT method

Using the BBT method means staying on top of your daily menstruation cycle readings so you know when and when not to have unprotected sex. Miscalculations or errors can sometimes happen, and that’s okay.

Have Julie on hand as a backup to the BBT method. Julie, an over-the-counter morning-after pill, delays the release of the egg from the ovary. Take it ASAP within 72 hours (3 days) of unprotected sex. Note: Julie is not effective if you’ve already ovulated. Julie is FDA-approved and legal in all 50 states. Pick it up at Target, CVS, Walmart, or order it through GoPuff (and get it in 30 minutes). No ID, credit card, or insurance is required.