How To Track Your Basal Body Temperature (BBT) for Hormone Health

Basal body temperature, or BTT, is your lowest body temperature at rest on any given day. For people who have periods, fluctuations in hormone levels – particularly the hormone progesterone – causes BTT to increase slightly. BTT tracking is a way of gauging your fertility window, letting you know what days of the month you’re most likely to get pregnant if you have sex. 

Other than family planning, tracking your BTT can help you more accurately link each stage of your cycle to how you’re feeling, confirm the timing of ovulation, and provide a more comprehensive view of your body’s natural rhythms. 

How Does Your Cycle Affect Basal Body Temperature?

Your BTT changes through each stage of your cycle, increasing after ovulation and decreasing right before ovulation. 

• Follicular phase: During the pre-ovulation (follicular) phase of the menstrual cycle, the average adult female basal body temperature typically falls around 97.0 – 97.5°F (36.1°C to 36.4°C). This is the beginning of your cycle, when estrogen dominates. 
• Ovulation: In the 1-2 days leading up to ovulation, there’s a small “dip” of about 0.5°F (0.2°C). This is the lowest BTT in your cycle. 
• Luteal phase: Progesterone levels rise as your body prepares itself for potential pregnancy, increasing resting body temperature by 0.4°F to 1.0°F (0.2°C–0.5°C) to a higher range of roughly 97.6°F to 98.6°F (36.4°C–37°C). 
• End of cycle: If pregnancy doesn’t occur, progesterone levels drop and BTT goes down. Your body sheds the unfertilized egg and uterine lining through what we call the “period.” If pregnancy does occur, BTT remains elevated.

Ovulation Fever vs. Illness

Do you ever feel like you’re coming down with the flu in the days leading up to your period? If you consistently feel achy, tired, and a little feverish during that time of the month, it may not just be an (in)conveniently timed cold – you may be experiencing “ovulation fever.” 

During the luteal phase, your body releases chemical messengers called prostaglandins. Prostaglandins are the same chemical messengers your body produces when you’re sick to combat bacteria and viruses. When you start your period, prostaglandin released in the uterus lining raises your body temperature, causing a low-grade fever. Prostaglandin combined with low estrogen levels and the rise in BBT due to ovulation may mimic flu-like symptoms such as fatigue and pain. 

PMS symptoms vary widely from person to person, and researchers are still finding out new things about “ovulation fever.” Keeping track of your hormonal cycle can help you figure out what times of month you’re most likely to feel tired, and whether or not you’re really sick. Remember: Cyclical body temperature changes are generally subtle, and if your body temperature spikes significantly or remains high past your period, it’s time to see a doctor.

Why Should I Track My BTT?

You may be thinking: I’m super careful about using condoms, and I’m not trying to get pregnant. Why should I track my BTT? 

Cycle tracking is a way of gauging your hormonal health, providing real-time data on hormonal fluctuations, which may be responsible for a range of symptoms, from irregular menstrual cycles and fatigue to weight changes and irritability. The average person experiences their period in a 21 to 35-day cycle, but this range (which is already quite broad) can vary.

BTT offers a more specific way to measure your bleeding patterns; If your periods have never been very regular, tracking your BTT helps you figure out when your next period will be and plan accordingly (you may want to reschedule a swimming outing on days after the “dip,” for example). 

BTT patterns may also be a predictor of hormonal health. For example, research shows that consistently low BTT readings correlate to an underactive thyroid. If your BTT patterns are all over the place, it could indicate hormonal conditions such as polycystic ovary syndrome (PCOS), stress-induced fluctuations, or perimenopause. Understanding your hormonal patterns can key you into potential problems and when to seek medical intervention. 

How To Track BTT: A Step-by-Step Morning Routine

Tracking your BTT requires consistency and a solid sleep schedule. Keep in mind that your basal body temperature can be affected by various factors, such as:

• Stress
• Illness or fever
• Not sleeping well, or sleeping more than usual
• Substances like alcohol
• Certain medications
• Travel, especially across time zones
• Gynecologic disorders

You’ll need a thermometer specifically designed to measure basal body temperature and measures to two decimal places for accuracy. Most BTT thermometers can be inserted under the tongue, though there are also vaginal or rectal options. Here’s how to track BTT:

1. Take your temperature at the same time each day, when you first wake up.

Our body temperature is the lowest right when we wake up in the morning, and increases with activity. Make sure to take your BTT immediately upon waking, before you sit up, drink water, or go to the bathroom. Always take your temperature using the same methods as close to the same time every day as possible.

2. Track the numbers.

Use a chart or tracking app to record your daily basal body temperature. There are several apps designed for this purpose which can provide useful visual aids.

3. Look for a pattern.

You should see a clear pattern emerge over a few months; ovulation should be occurring on the days your BTT rises slightly. Be patient – self-measuring is prone to human error, and it may take some time before you get a regular pattern. 

Hormones Should Be Cyclical, Not Exact

We’re not robots, and sometimes a night out or stress can confuse your readings. However, if you’re fairly consistent in your habits, and spot clear changes in your BTT over months of tracking, it may be a sign of hormonal disturbance. Knowing how your hormones impact the way you feel can help you understand your body and provide valuable insight to your doctor in case of concern.