04/17/2025 2:27 p.m.

Understanding Your Basal Body Temperature with a BBT Thermometer for Fertility Tracking

Dr. Niels van de Roemer
Dr. Niels van de Roemer Medical Adviser

BBT for Natural Family Planning

Basal body temperature is an important part of the female cycle. It is used to determine fertile and non-fertile days as well as ovulation. BBT changes during the cycle in line with the hormones and thus shows you your ovulation, the different cycle phases, and your menstruation1.

Using a high-quality BBT thermometer to track these measurements provides valuable information to increase or rule out the likelihood of pregnancy, allowing you to get to know your body's unique rhythm.

A central part of your cycle

What is Basal Body Temperature?

During the night, your body comes to rest and reaches its lowest temperature point. Because this temperature is measured every morning directly after waking up, it is also called the "wake-up temperature." Shortly after waking and making your first movements, your core body temperature rises again.

Due to the measurable influence of the sex hormone progesterone on the basal body temperature, your wake-up temperature varies according to a recurring pattern over the course of your cycle. With a precise BBT thermometer, you can accurately determine the follicular phase, your ovulation, and the subsequent luteal phase based on a slight rise in temperature 1.

  • Follicular phase (before ovulation): The temperature is comparatively low.

  • Luteal phase (after ovulation): The temperature rises slightly - typically by around 0.2 to 0.45 °C 1.

A valuable indicator of your health and fertility

Why should you track your BBT?

If you want to have children or for natural family planning

Measuring your basal body temperature lets you take your fertility into your own hands, in a self-determined way and completely without hormones. By recognising your ovulation, you can shape your family planning to fit your life4.

This is especially valuable for couples who want children. It supports a reliable assessment of ovulation and fertile days and thus helps increase the chances of conception. Knowing your cycle phases lets you plan intimacy during the fertile phase, and cycle monitoring provides a sound basis for medical counselling, for example during fertility treatment4,7

If you want to avoid pregnancy, measuring your basal body temperature can be a natural and hormone-free way to better understand your cycle. By recognising your fertile days, you will know exactly when you need to use a barrier method of contraception 9.

For a monthly health check

Basal body temperature is your fifth vital sign and an important indicator of health, comparable to breathing or heart activity. A regular, healthy cycle signals normal hormonal function. An unusual BBT pattern, for example unusually high or low temperatures or a missing rise after ovulation, can be worth discussing with your doctor5. Examples:

  • Thyroid disorders
  • PMOS (formerly PCOS)
  • Luteal insufficiency
  • Missing cycle

For a better understanding of your body and psyche

Measuring your basal body temperature not only gives you a deeper understanding of the physical processes of your cycle, but also promotes your psychological well-being. By regularly measuring your basal body temperature, you can live in sync with your cycle:

  • How hormonal changes affect your mood.
  • How your energy levels fluctuate during your cycle.
  • How you can organise self-care in harmony with your cycle.
  • How stress and life changes affect your basal body temperature.

By recognising all these connections, you create a holistic understanding of your physical and emotional well-being.

How can you take your basal body temperature?

Basal body temperature is measured right after waking, while lying down, using a special BBT thermometer or a fertility tracker such as Daysy or Lady-Comp3,8

This is how temperature measurement with Daysy works:

  1. As soon as you wake up, take Daysy in your hand, place the sensor under your tongue as far back as possible and close your mouth.
  2. After less than 60 seconds, you will hear a soft tone and Daysy will show you your fertility status for the next 24 hours.
  3. Simply connect Daysy to the DaysyDay app and view your cycle data in detail using the temperature curve or cycle summaries.
Progesterone controls basal body temperature

How does BBT change during the menstrual cycle?

The cyclical temperature fluctuations are approximately 0.36 to 0.81 °F (0.2 to 0.45 °C), triggered by progesterone (the corpus luteum hormone). This subtle rise right after ovulation can be tracked with a precise BBT thermometer1.

With the temperature curve, you can clearly follow the individual phases of your cycle and gain exciting insights into your body.

How does BBT indicate ovulation?

In the first phase (follicular phase), the basal body temperature stays relatively constant. After ovulation, the body produces more progesterone, which acts on thermoregulation and raises the basal body temperature.

To identify ovulation, you look for this rise ("ovulation spike") in the temperature curve. Cycle trackers like Daysy or Lady-Comp indicate your ovulation directly on the device1,2,6.

How does BBT indicate your period?

After ovulation, the temperature stays elevated for around 12 to 16 days until menstruation. If the egg is not fertilised, progesterone drops, and the subsequent fall in temperature signals that your period is about to start5.

How does BBT show you that you are pregnant?

The basal body temperature method can also help detect an early pregnancy. Following ovulation, a rise in basal body temperature that lasts for 18 or more days may be an early indicator of pregnancy10. In early pregnancy, the hormone hCG "rescues" the corpus luteum, so it keeps producing progesterone and your basal body temperature stays elevated instead of dropping13.

Some people also notice a further slight rise around this time (a "triphasic" pattern), or a brief one-day "implantation dip" about 7 to 10 days after ovulation, but neither is said to be a reliable sign of pregnancy 11.

Implantation itself usually occurs 6 to 12 days after ovulation, most often on days 8 to 1012. If your period is late and your temperature stays elevated, take a pregnancy test to confirm.

No matter what phase of life

Basal body temperature tracking is the key to understanding your personal cycle.

Measuring your basal body temperature allows you to understand the cycle-related processes in your body and gives you control over your reproductive health. From planning a pregnancy to early detection of hormonal imbalances and promoting your emotional well-being, cycle tracking with a reliable BBT thermometer supports you in all situations.

FAQs

Can I use a regular fever thermometer to track my cycle?

No, a standard fever thermometer is not precise enough for ovulaiton tracking 3. To accurately detect the subtle shift in temperature that occurs after ovulation (often as small as 0.2°C or 0.4°F), you need a specific BBT thermometer. These thermometers measure to two decimal places (e.g., 36.65°C), whereas regular thermometers usually only measure to one. Daysy is a highly sensitive fertility monitor that functions as a BBT thermometer to ensure you get the necessary accuracy.

When is the best time to measure my temperature?

You must measure immediately upon waking up and before you get out of bed. Do not speak, drink water, or check your phone before measuring. Basal body temperature is your body's temperature at complete rest, so any physical activity can cause it to rise. Keep your BBT thermometer Daysy within arm's reach on your nightstand so you don't have to move to get it.

What can influence my basal body temperature reading?

Basal body temperature is a sensitive sign of your body and can be influenced by various factors, including stress, lack of sleep, alcohol consumption, medication and, in particular, illnesses with fever. Such external influences can lead to short-term fluctuations. If you don't feel well in the morning, it makes sense to skip taking your temperature. Certain medications such as sleeping pills, painkillers, psychotropic drugs or hormone preparations can also affect your temperature.

However, if you use Daysy or Lady-Comp, you don't need to worry about this, because thanks to the intelligent algorithm, our devices will always tell you with 99.4 % accuracy whether you are fertile or not. Even if you have a fever, the DaysyDay app will automatically tell you that you should take your temperature with a standard thermometer. Conveniently, temperatures above 37.8°C or below 35°C are automatically excluded from the calculations by the algorithm.

How reliable is the basal body temperature method?

Daysy combines BBT with a self-learning algorithm validated on over 10 million cycles to determine your non-fertile days with 99.4% accuracy (van de Roemer et al., 2021).

Can I use the basal body temperature with an irregular cycle?

Yes, you can also use the basal body temperature method very well for irregular cycles. It helps you to recognise the time of your ovulation and your period based on the rise in temperature. With Daysy or Lady-Comp, you can use the intelligent algorithm to get a reliable prediction of your ovulation time even with irregular cycles. - Please note that irregular cycles may result in additional fertile or potentially fertile indications during the longer cycles.

Do I always have to measure at the same time?

No. It is important that you have always slept or rested for at least an hour before taking your temperature and that you take your temperature immediately after waking up, before you get up and become active.

How long does it take to see a pattern?

It can take a cycle or two to get used to the routine and for you to start recognizing the patterns in your chart. However, a smart device like Daysy begins learning your cycle from day one. While a manual BBT thermometer requires you to interpret the curve yourself, Daysy analyzes the data instantly to give you a fertility status.

Learn more

Your guide to interpreting basal temperature charts and recognizing patterns – from typical temperature curves to potential signs of health changes.

Temperature Charts

Discover all about PCOS: its symptoms, diagnosis, treatment options, and how tracking your cycle can support women living with this condition.

PMOS (formerly PCOS)

The thyroid is a central regulator of the human metabolism. Its importance for the female cycle and fertility is often underestimated.

Thyroid & Its Influence

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