Basal Body Temperature (BBT) - why you should track it and how diet affects it

Reading time: 6 min

If you're planning a pregnancy and want to understand your body better, tracking your basal body temperature (BBT) is one of the simplest and cheapest tools available to you. You don't need expensive tests or doctor's appointments - just a thermometer and a few minutes each morning.

In this article I'll explain what BBT is, how to measure it correctly, what it tells you about your cycle, and how your diet can affect your temperature and fertility.

What is basal body temperature?

Basal body temperature is the lowest temperature your body reaches at rest - measured immediately after waking, before getting out of bed, before having a drink, before going to the bathroom. It's your body's temperature in a state of complete rest.

For most women, BBT is around 36.1-36.4°C before ovulation. After ovulation it rises by approximately 0.2-0.5°C (to around 36.4-36.8°C) and stays elevated until your next period.

This rise is caused by progesterone - the hormone your body produces after ovulation. Progesterone literally "warms" your body, preparing it for a potential pregnancy.

Source: Barron ML, Fehring RJ. "Basal body temperature assessment: is it useful to couples seeking pregnancy?" MCN Am J Matern Child Nurs. 2005;30(5):290-296.

Photo by Clarisse Meyer on Unsplash

What does BBT tell you about your cycle?

Confirming ovulation

If you see a clear temperature rise that stays elevated for at least 3 days, that's a strong signal that ovulation has occurred. This is the most important piece of information BBT provides.

Important: BBT confirms that ovulation HAS already happened. It doesn't predict it in advance. That's why BBT works best combined with cervical mucus observation (which signals APPROACHING ovulation) and LH tests.

Detecting anovulatory cycles

If your temperature chart is flat throughout the entire cycle with no clear rise, it may mean ovulation didn't occur (an anovulatory cycle). This happens more often than you might think - even in healthy women occasionally. However, if it happens regularly, it's worth speaking to your doctor.

Luteal phase length

The luteal phase is the period from ovulation to your period. It should last 10-16 days. If your luteal phase is shorter than 10 days (meaning your temperature drops and your period arrives too soon after ovulation), this can make embryo implantation difficult. It's something worth discussing with your gynaecologist.

Early pregnancy

If your temperature stays elevated for more than 18 days after ovulation, there's a good chance you're pregnant. BBT doesn't replace a pregnancy test, but it can be the first clue.

Source: Su HW, et al. "Detection of ovulation, a review of currently available methods." Bioeng Transl Med. 2017;2(3):238-246.

How to measure BBT correctly

You'll need:

• A thermometer accurate to 0.01°C (two decimal places). A standard clinical thermometer with one decimal place isn't precise enough. Dedicated BBT thermometers cost around £5-15 / 20-50 PLN.
• An app or notebook to record your results. In our app you can enter BBT directly in the cycle calendar.

Measurement rules:

1. Measure every morning at the same time - set an alarm. Even a 30-minute difference can affect the reading.
2. Measure BEFORE getting out of bed - before sitting up, before having a drink, before going to the loo. Place the thermometer on your bedside table the evening before.
3. Measure after at least 3 hours of uninterrupted sleep - if you got up in the night, the reading may be inaccurate.
4. Measure orally (under the tongue) for at least 3 minutes. Some women measure vaginally - it's more accurate but not comfortable for everyone.
5. Record immediately - in the app or on paper. Don't rely on memory.
6. Note any disrupting factors - alcohol the previous evening, illness/fever, too little sleep, stress, travel. These can raise your temperature.

What to expect:

• The first 1-2 cycles are a learning phase. Don't expect a perfect chart straight away.
• Your chart won't be perfectly smooth - daily fluctuations are normal. You're looking for the TREND: lower temperatures before ovulation, higher after.
• After 2-3 cycles you'll see your pattern and be able to anticipate ovulation based on your history.

Source: Steward K, Raja A. "Physiology, Ovulation And Basal Body Temperature." StatPearls. 2023.

What if my temperature is higher than normal?

Many women worry when they see 37°C or above on their thermometer. It doesn't always mean something is wrong - but it's worth understanding what might be behind it.

37°C after ovulation - usually normal

The normal BBT range after ovulation is approximately 36.4-37.0°C. If your luteal phase temperature is around 37°C but is clearly lower before ovulation (e.g. 36.2-36.4°C) - this is simply your individual norm. Some women naturally have a higher body temperature, and this isn't a cause for concern as long as your chart shows a clear biphasic pattern (lower before ovulation, higher after).

37°C before ovulation - worth investigating

If your temperature is close to 37°C already in the follicular phase (before ovulation), when the norm is around 36.1-36.4°C, this may suggest:

• Hyperthyroidism - the thyroid is producing too much T3 and T4, which speeds up metabolism and raises body temperature
• Inflammation in the body - e.g. a hidden infection
• External factors - stress, sleep deprivation, intense evening exercise

37.5°C - almost always a disrupting factor

A temperature of 37.5°C in the morning is in most cases the result of something from the previous day or night:

• Alcohol (even a single glass of wine)
• A cold or infection (even without obvious symptoms)
• Too little sleep or restless sleep
• Intense physical exercise in the evening
• Measuring later than usual (even a 30-minute difference)

Mark this reading as "disrupted" in the app and don't include it in your chart analysis. One elevated point doesn't ruin your entire cycle - we simply skip it.

When to be concerned: If your temperature is consistently elevated (above 37°C) for many days without an obvious cause, particularly in the follicular phase, ask your GP to check your thyroid (TSH, FT3, FT4) and inflammatory markers (CRP).

Photo by Sincerely Media on Unsplash

Thyroid and body temperature - a brief overview

Your thyroid is your internal thermostat. Thyroid hormones (T3 and T4) regulate the rate of your metabolism, and therefore the amount of heat your body produces:

• Hypothyroidism (underactive thyroid) - too few hormones = metabolism slows down = BBT consistently low, often below 36.0°C even after ovulation
• Hyperthyroidism (overactive thyroid) - too many hormones = metabolism speeds up = BBT consistently elevated, above 37°C

Both conditions affect fertility. Hypothyroidism is significantly more common in women and can cause irregular cycles, lack of ovulation, and difficulty conceiving. This is a topic for a separate article - I'll be writing about it soon, as it deserves a thorough discussion.

Source: Barrera FJ, et al. "Hypothyroidism and Hyperthyroidism." In: Endotext. 2023. Source: Poppe K, et al. "Thyroid disease and female reproduction." Clin Endocrinol. 2007;66(3):309-321.

How diet affects BBT and fertility

Here's where we connect temperature with nutrition - my area of expertise. Diet affects BBT and hormonal health in several ways:

1. Progesterone needs healthy fats

Progesterone (the hormone responsible for the BBT rise after ovulation) is produced from cholesterol. Diets too low in fat can disrupt progesterone production, which shows up as:

• A short luteal phase (fewer than 10 days of elevated BBT)
• A low temperature rise after ovulation (less than 0.2°C)
• Spotting before your period

What to eat: Avocado, olive oil, nuts, seeds, oily fish (salmon, sardines), whole eggs. Don't avoid fat - your hormones need it.

Source: Mumford SL, et al. "Dietary fat intake and reproductive hormone concentrations and ovulation in regularly menstruating women." Am J Clin Nutr. 2016;103(3):868-877.

2. Your thyroid controls body temperature

Thyroid hormones (T3, T4) regulate metabolism and body temperature. An underactive thyroid can cause:

• Consistently low BBT (below 36.0°C even after ovulation)
• Irregular cycles
• Difficulty conceiving

What to eat for your thyroid:

• Iodine - sea fish, milk, yoghurt, eggs (see my article on iodine)
• Selenium - Brazil nuts (just 2-3 per day is enough!), fish, eggs
• Zinc - pumpkin seeds, beef, cashew nuts

If your BBT is consistently low, ask your GP for a TSH test.

3. Too few calories disrupt ovulation

Restrictive diets and excessive calorie deficits can "switch off" ovulation. Your body interprets an energy shortage as a signal that it's not a safe time for pregnancy. This shows up as:

• A flat BBT chart (no rise = no ovulation)
• Irregular or missing periods
• So-called hypothalamic amenorrhoea (HA)

What to do: Eat regularly (3 meals + 1-2 snacks), don't skip meals, don't obsessively count calories. Your body needs a minimum of 1800-2000 kcal per day to maintain regular cycles.

Source: Fontana R, Della Torre S. "The Deep Correlation between Energy Metabolism and Reproduction." Front Endocrinol. 2016;7:43.

4. Insulin resistance and PCOS

High insulin levels (common in PCOS) disrupt the balance of oestrogen and progesterone. On a BBT chart this can look like:

• Chaotic temperatures with no clear pattern
• Long cycles (over 35 days)
• Multiple "false starts" of temperature rises

What to eat: A low glycaemic index diet. Whole grains instead of white bread, protein and fat with every meal (to stabilise blood sugar), plenty of vegetables, limited added sugar.

Source: Barrea L, et al. "Source and amount of carbohydrate in the diet and inflammation in women with polycystic ovary syndrome." Nutr Res Rev. 2018;31(2):291-301.

5. Alcohol and caffeine

Alcohol can elevate BBT (even a single glass of wine in the evening can raise your temperature the following morning). Excess caffeine (over 200 mg/day) can affect cortisol levels, which in turn interact with progesterone.

What to do: Mark the days you drank alcohol in the app - this helps you interpret fluctuations on your chart. Limit coffee to 1-2 cups per day.

Photo by Joseph Gonzalez on Unsplash

How to use BBT in our app

In the cycle calendar you can enter daily: 

• Temperature (accurate to 0.01°C)
• Cervical mucus (dry / sticky / creamy / egg white)
• Notes (alcohol, illness, stress, poor sleep - disrupting factors)

The app displays a temperature chart below the calendar, so you can see the trend from your entire cycle at a glance.

Remember: Our calendar is a tracking tool, not a diagnostic tool. If you notice concerning patterns (no temperature rise, a very short luteal phase, consistently low BBT), speak to your doctor or gynaecologist.

Your tasks for this week

1. Buy a BBT thermometer - accurate to 0.01°C. Available at pharmacies or online for around £5-15 / 20-50 PLN.
2. Start measuring tomorrow morning - place the thermometer on your bedside table tonight.
3. Record daily in the app - temperature + mucus + notes.
4. Don't stress about the results - the first 1-2 cycles are for learning. You'll see the pattern over time.

This article is for educational and informational purposes only. It does not replace medical or dietary advice. If you need an individualised plan - book a consultation.

"Health is the greatest human value"

Oksana Rozponczyk Clinical Dietitian | AnaskoMed Clinic

References:

1. Barron ML, Fehring RJ. Basal body temperature assessment: is it useful to couples seeking pregnancy? MCN Am J Matern Child Nurs. 2005;30(5):290-296.
2. Su HW, et al. Detection of ovulation, a review of currently available methods. Bioeng Transl Med. 2017;2(3):238-246.
3. Steward K, Raja A. Physiology, Ovulation And Basal Body Temperature. StatPearls. 2023.
4. Mumford SL, et al. Dietary fat intake and reproductive hormone concentrations and ovulation in regularly menstruating women. Am J Clin Nutr. 2016;103(3):868-877.
5. Fontana R, Della Torre S. The Deep Correlation between Energy Metabolism and Reproduction. Front Endocrinol. 2016;7:43.
6. Barrea L, et al. Source and amount of carbohydrate in the diet and inflammation in women with polycystic ovary syndrome. Nutr Res Rev. 2018;31(2):291-301.
7. Bull JR, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med. 2019;2:83.
8. Barrera FJ, et al. Hypothyroidism and Hyperthyroidism. In: Endotext. 2023.
9. Poppe K, et al. Thyroid disease and female reproduction. Clin Endocrinol. 2007;66(3):309-321.