What Is a Normal Basal Body Temperature?
There's no single fixed "normal" basal body temperature (BBT). It's simply your body temperature at rest, measured immediately after waking up before any movement or activity.
According to Dr. Niels van de Roemer (PhD), Daysy's medical expert: "During sleep, a temperature minimum is established in the body core (the base), which is called basal temperature (formerly also ovarian temperature). In practice, the core body temperature upon waking is often referred to as the basal temperature."
Your body resets its temperature during sleep, which is why BBT needs to be measured first thing in the morning. If you look at your BBT chart, you'll notice it isn't constant but moves within a narrow range, following a biphasic pattern that reflects the two phases of your menstrual cycle. Your hormones are responsible for this pattern.
Each menstrual cycle has two key phases: the follicular phase and the luteal phase. Day one of your period marks the beginning of the follicular phase. During this time, multiple follicles develop in your ovaries. Eventually, one matures and is released at ovulation, which ends the follicular phase.
During the follicular phase, oestrogen is the dominant hormone, and it keeps your basal temperature slightly lower. Most women record BBT readings between 36.1°C and 36.4°C (97°F and 97.5°F) before ovulation.
Right after ovulation, you'll notice a slight upward shift of around 0.2°C (0.9°F) on your BBT chart. This marks the start of the luteal phase, named after the corpus luteum, the gland that forms from the follicle after the egg is released. The corpus luteum produces progesterone, which acts on the hypothalamus and thyroid to raise your baseline temperature. This is why your BBT runs a little higher in the second half of your cycle.
What If You See Two Peaks on Your BBT Chart?
Some women notice two temperature rises in their BBT chart. The first typically occurs right after ovulation, while the second may appear around 6 to 12 days later. This second rise may be related to implantation of the embryo in the uterus, and a triphasic BBT curve like this can sometimes be an early sign of pregnancy. That said, implantation doesn't always produce a second peak, so a pregnancy test is the only way to confirm.
What If There's No Peak at All?
A monophasic BBT curve, where there's no temperature rise across the cycle, usually indicates that ovulation didn't occur. Without ovulation, the corpus luteum doesn't form, so there's no progesterone surge to warm up the second phase of your cycle.
Research suggests that 10 to 15% of all cycles are anovulatory (Study). They're more common at the beginning of puberty and approaching menopause. If you experience anovulatory cycles more than three times a year, it's worth speaking with your doctor. PCOS, stress, and thyroid disorders are among the factors associated with anovulation.
If you're using the Daysy fertility tracker, you can see when and how often ovulation isn't occurring. If it happens regularly, discuss it with your healthcare provider, particularly if you're trying to conceive.
Does Basal Body Temperature Rise Before a Period?
Your BBT increases by around 0.2 to 0.3°C (0.4 to 0.5°F) immediately after ovulation due to progesterone from the corpus luteum. It stays elevated throughout the luteal phase, then begins to drop a few days before your period arrives.
Here's How It Works:
Your menstrual cycle has two key stages: the follicular phase and the luteal phase.
The follicular phase begins with your period and ends at ovulation. Your BBT is lower during this phase because oestrogen is the dominant hormone, and it promotes heat loss by acting on the brain.
After ovulation, the luteal phase begins. Oestrogen dips and progesterone rises as the corpus luteum becomes active. Progesterone prevents the breakdown of the uterine lining and raises your body's baseline temperature, which is why you see a BBT peak after ovulation.
Around day 10 of the luteal phase, progesterone production starts to decline. This causes your BBT to begin falling roughly one to two days before your period starts.
If your BBT stays elevated for more than 18 days after ovulation, it's worth taking a pregnancy test. During early pregnancy, HCG from the placenta sustains the corpus luteum, keeping progesterone levels up and your BBT high. Prolonged elevated BBT is not a definitive pregnancy indicator on its own, but it's a useful signal.
What Is the Basal Body Temperature Range During Pregnancy?
Pregnancy typically keeps BBT elevated above pre-ovulation levels due to sustained progesterone production. The rise isn't dramatic, averaging around 0.2 to 0.3°C (0.4 to 0.5°F), so your temperature won't reach fever levels. A BBT that remains elevated for more than 18 days after ovulation can be an early indicator of pregnancy, though a test is still needed to confirm.
How Pregnancy Affects Your BBT:
After ovulation, BBT peaks as you enter the luteal phase. If pregnancy occurs, the corpus luteum stays active for roughly the first trimester. HCG from the placenta maintains the corpus luteum, which continues producing progesterone to support the uterine lining and sustain the pregnancy. This also keeps your BBT elevated.
Around week 10, the corpus luteum begins to shrink, and by week 12 it stops producing progesterone. Progesterone production then shifts to the placenta. Some women notice a slight BBT change around this time, though BBT generally remains elevated throughout pregnancy.
Should You Track Your BBT During Pregnancy?
That's entirely your choice. Your BBT chart can help you visualise early pregnancy progress, particularly in the first trimester. According to Dr. van de Roemer, some fluctuation in BBT during pregnancy is normal and doesn't necessarily indicate a problem.
Most women set their fertility tracker aside during pregnancy and return to it after giving birth. Around six weeks postpartum is a good time to resume, which is roughly when your first period may return. That said, the first period after childbirth can arrive anywhere between five weeks and a year after delivery.
How to Check Your Basal Body Temperature
Measure your BBT each morning immediately after waking, before getting up or doing anything else. Place a thermometer under your tongue and wait for the reading. Specialised devices like the Daysy fertility tracker are far more accurate than standard thermometers for this purpose, as their precision sensors detect the subtle temperature shifts that matter for cycle tracking.
Timing is everything. Your body reaches its lowest temperature during sleep, so any movement, talking, or even a sip of water before measuring can push the reading up. Keep your Daysy on your bedside table so you can reach it without getting out of bed. Aim for at least 3 to 4 hours of sleep beforehand for a reliable reading.
Here's how to use your Daysy fertility tracker each morning:
- Activate: Press the button once to turn Daysy on, then again to activate it. Blue rotating lights indicate it's ready to measure.
- Insert: Place the sensor under your tongue as far back as possible and close your mouth.
- Wait: Measurement takes less than 60 seconds. Blue swirling lights mean it's still recording. Two beeps signal that it's done. If you're unsure whether your temperature was recorded, try reactivating the device. If it was already taken, it won't re-enter measurement mode.
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Read the lights:
- Red: Fertile or possibly fertile
- Red flashing: Possible ovulation
- Green: Infertile days
- Yellow: Daysy is still learning your cycle, or your cycle is fluctuating
- Red, green, and yellow blinking together: Possible pregnancy
You'll see more red and yellow days in your first few cycles as Daysy learns your patterns. This is normal and improves over time. Missing the occasional day won't affect accuracy. Daysy is backed by over 30 years of research to help you take confident control of your fertility.
