Understanding Temperature Charts and How To Read Them

Every menstrual cycle consists of two primary phases: the follicular phase and the luteal phase.

• Follicular Phase (Day 1 – Ovulation): This phase begins on the first day of your period. During this time, multiple ovarian follicles begin to mature, but usually, only one will release an egg during ovulation. On your BBT chart, temperatures are slightly lower during this phase due to the influence of estrogen, which has a temperature-lowering effect. Most women record BBT readings between 97°F (36.1°C) and 97.5°F (36.4°C) before ovulation³.

• Luteal Phase (Post-Ovulation): After ovulation, you'll notice a slight upward shift in your BBT, typically around 0.4°F to 0.5°F (0.2°C). This phase begins when the corpus luteum, the gland formed from the released follicle, starts producing progesterone. Progesterone not only maintains the uterine lining but also causes your temperature to rise slightly by acting on the hypothalamus and thyroid³.

A typical cycle curve spans about 28 days.

• Ovulation usually occurs between day 12 and day 13, indicated by a distinct rise in basal temperature.
• The luteal phase (after ovulation) lasts for about 15 days, ending with the onset of menstruation.

This temperature rise is caused by the hormone progesterone, which increases body temperature slightly after ovulation.

Note: Cycle lengths can vary. Longer or shorter cycles will generally have a later or earlier ovulation, respectively.

Some women experience a slight drop in BBT right before ovulation. This last lower temperature reading, compared to the days before, is a result of an estrogen surge.

Tip: This is considered an ideal day for intercourse if you’re trying to conceive, as it indicates that ovulation is imminent.

A consistently higher-than-normal average BBT might indicate a condition such as hyperthyroidism (overactive thyroid).

Other symptoms may include³:

• Longer menstrual cycles
• Light or scanty bleeding
• Increased nervousness or restlessness
• Rapid or excessive sweating

Important: While a higher curve can be a clue, only a doctor can confirm thyroid-related conditions.

On the other hand, a low average BBT may indicate hypothyroidism (underactive thyroid).

Possible signs include³:

• Missed periods or amenorrhea
• Heavy or prolonged bleeding
• Irregular cycles
• Cycles without ovulation (no temperature rise)
• Short luteal phases
• Noticeable temperature fluctuations

If your BBT drops below 35°C (95°F), apps like DaysyDay will automatically flag this. Note: Medical evaluation is necessary for proper diagnosis.

Sometimes, a reading may be significantly higher or lower than the rest. These outliers can happen due to:

• Lack of sleep or oversleeping
• Alcohol consumption
• Fever or illness
• Stress or emotional strain
• Incorrect measuring techniques

The Daysy and Lady-Comp fertility trackers automatically detect such anomalies, often marking them as yellow (uncertain) or red days.

A body temperature of 38°C (100.4°F) or higher indicates fever, which will naturally cause your BBT to rise abnormally. The DaysyDay app recommends using a standard thermometer if this happens.

• Temperatures above 37.8°C (100°F) or below 35°C (95°F) are automatically flagged as invalid readings.

Rule of Thumb: If you wake up feeling unwell, hungover, or overtired, pause BBT measurements until you recover.

When you measure your basal body temperature (BBT) with a cycle computer, you gain valuable insights into your menstrual cycle. However, no cycle is exactly the same, and not every temperature measurement is flawless. Small deviations, also known as temperature outliers, are a normal part of the process.

What Are Temperature Outliers?

Your temperature curve isn't influenced solely by your hormones; external factors, called disturbance factors, can also affect it. These can distort individual readings and lead to irregular spikes or drops on your chart. Typical causes include:

• Insufficient or disrupted sleep
• Waking up unusually early
• Shift work
• Unusual alcohol consumption
• Stress, emotional strain, or excitement
• Changes in environment (travel, holidays, climate changes)
• These influences are perfectly normal and occur for nearly every user at some point.

How Does the Cycle Computer React to Temperature Fluctuations?

Studies show that the algorithm used by Daysy and Lady-Comp is very sensitive to such fluctuations, and this is actually beneficial².

• If your temperature curve remains relatively stable, showing few outliers (see figure a), Daysy can calculate more green days (not fertile) – averaging around 56%.
• However, if fluctuations are more pronounced (see figure d), the number of green days may drop to about 43%, while yellow days (uncertain) increase.

This means that Daysy and Lady-Comp adapt to your individual cycle patterns, even if they are irregular.

What About Days When You Don’t Measure?

Missing a day or two isn’t a problem – nobody measures their BBT 100% of the time. Still, the rule of thumb is: The more consistently you measure, the more accurate your cycle computer will be.

An analysis of over 47,000 cycles revealed that²:

• 53.1% of users who measured on 80–100% of their cycle days received an average of:
• 41% red days (fertile)
• 42% green days (not fertile)

This clearly shows that regular measurements help define fertile days more precisely. Even so, Daysy’s algorithm is designed to work with fewer data points and will flexibly adapt to your lifestyle and routine.

Some women observe two distinct temperature rises in their cycle. The first peak occurs right after ovulation, while the second may appear 6–12 days later, potentially indicating embryo implantation.

This triphasic BBT curve can be an early sign of pregnancy, but not all women experience this pattern. The only way to confirm pregnancy is with a test.

Another possible pattern is the monophasic curve, where there’s no clear temperature shift. This typically suggests that ovulation did not occur (anovulation). Without ovulation, the corpus luteum does not form, and progesterone levels remain low, preventing the usual rise in BBT.

Note: About 10–15% of all cycles are anovulatory³, which is more common during puberty or near menopause. Conditions such as PMOS (formerly PCOS), stress, or thyroid disorders can also lead to anovulatory cycles. If this happens more than three times a year, consult your doctor.

Tracking your cycle with tools like the Daysy fertility tracker can help identify patterns of missed ovulation, which is especially important if you’re trying to conceive.

During pregnancy, progesterone levels remain high, which causes BBT to stay elevated. On average, your BBT may rise by 0.4–0.5°F (0.2–0.3°C) above your pre-pregnancy baseline. A sustained BBT elevation of more than 18 days after ovulation can be an early pregnancy indicator³, though a test is necessary for confirmation.

How Pregnancy Affects BBT:

• After ovulation, BBT increases as part of the luteal phase.
• If pregnancy occurs, the corpus luteum continues to produce progesterone during the first trimester, maintaining the higher temperature.
• Around week 10, the placenta begins to take over progesterone production, and by week 12, the corpus luteum dissolves. Some women may notice a slight dip in BBT during this transition, but for most, it stays elevated throughout pregnancy.

Should You Track BBT During Pregnancy?

Tracking BBT during pregnancy is optional. It can help visualize early changes, but fluctuations do not necessarily indicate problems. After childbirth, you can resume tracking with Daysy or Lady-Comp approximately 6 weeks postpartum, which aligns with the return of your first period (though this can vary between 5 weeks to 1 year after delivery).

After ovulation, progesterone raises your BBT and keeps it elevated for approximately 12–16 days. A few days before your period begins, the corpus luteum breaks down, progesterone levels drop, and your BBT falls.

Quick Breakdown:

• Follicular Phase: Lower BBT, dominated by estrogen.
• Ovulation: Sharp temperature rise due to progesterone.
• Luteal Phase: Higher BBT, lasting until menstruation starts.
• Before Period: BBT falls as progesterone declines.

If your BBT stays elevated for more than 18 days, consider taking a pregnancy test.