Basal Body Temperature: How to Measure BBT to Detect Ovulation

What is basal body temperature?

Basal body temperature (BBT) is the lowest body temperature attained during a period of rest, and it’s particularly useful in estimating the day of ovulation. A BBT graph shows ovulation after it has occurred, not before, so you could use Flo's online ovulation calculator to predict when you might ovulate, and your basal body temperature data to confirm it.

To determine the best time for sexual intercourse, log and monitor your BBT for a minimum of two to three menstrual cycles. An ovulation pattern will gradually emerge, allowing you to plan accordingly for the days leading up to the next temperature increase.

How does the BBT сhart change after ovulation?

The release of an egg from the ovaries is accompanied by a rise in progesterone production and a spike in basal body temperature. That’s why BBT tracking is a fairly effective way to confirm ovulation. With daily monitoring of BBT, an increase of 0.5–1.0 degrees Fahrenheit (0.3 to 0.6 degrees Celsius) should be detected on the day after ovulation.

Using BBT to predict ovulation

BBT-based tracking is a simpler alternative to a gynecologic ultrasound, but it needs to be measured every day for several cycles with no breaks. Basal body temperature usually drops right before ovulation, then increases rapidly. It’s helpful to keep the following things in mind:

• During the follicular phase of the cycle, BBT stays in the lower range, generally between 97 and 98 degrees Fahrenheit (36.1 to 36.6 degrees Celsius) until approximately one day before ovulation, when BBT reaches its lowest point.
• After ovulation, the corpus luteum begins to secrete progesterone. The basal body temperature increases by 0.5–1.0 degrees Fahrenheit (0.3–0.6 degrees Celsius) and plateaus throughout the luteal phase. In the late luteal phase, when the corpus luteum regresses and serum progesterone level decreases, the BBT returns to the lower range either one to two days before or just at the onset of menstrual bleeding.

If a fertilized egg fails to implant, progesterone levels and basal body temperature both return to normal before menstruation.

Why measure basal body temperature instead of normal body temperature?

Fertility experts agree that monitoring basal body temperature is the easiest, most affordable method of detecting ovulation. It’s ideally measured in the morning after at least three to four hours of sleep and prior to any physical activity. 

Throughout the day, BBT fluctuates due to stress, cold, heat, exercise, food consumption, etc. A temperature that is unaffected by these external factors is regarded as the most reliable.

When to check BBT

When trying to conceive, take your basal body temperature every morning before getting out of bed. Creating a comprehensive BBT chart can assist you in:

• Understanding the processes your body is undergoing
• Determining your most fertile days
• Predicting menstruation
• Investigating potential hormonal imbalances and reproductive issues

Also, a BBT graph can tell you on which days to refrain from having unprotected sex to avoid conception. Of course, this approach works best when the menstrual cycle is stable, periods are regular, and measurements are accurate. However, several factors (e.g., stress, illness, lack of sleep) make it less than foolproof.

How to check BBT accurately

Basal body temperature data is useful for predicting ovulation days. Follow these steps to get an accurate BBT reading:

• Take your basal body temperature every morning at the same time.
• Don’t get out of bed, talk, or engage in any other activities before checking your temperature.
• Always utilize the same type of measurement (orally, vaginally, or rectally). 
• Use the same thermometer every time.
• Make sure you’ve had at least three hours of sleep prior to checking your BBT.
• Log all BBT data on paper or with an app like Flo for at least two to three cycles.

Additional things to remember:

• Of the three options available, rectal readings are considered the most reliable.
• When obtaining BBT orally, put the thermometer underneath the tongue and hold it in place. Keep your mouth closed for at least five minutes (if it’s mercury) or until you hear the signal (if it’s electronic).
• If you opt for the vaginal or rectal methods, the waiting period for a mercury thermometer is only three minutes.
• Never check your BBT in the underarm region, as it won’t provide an accurate reading.

Temperature fluctuations

BBT values obtained over a menstrual cycle should be entered into a chart that’s divided into two phases. The vertical dividing line corresponds to ovulation, with your cycle’s first and second phases on either side of the line. 

Changes in basal body temperature seem to result from progesterone’s impact on the body’s thermoregulatory center, as well as fluctuations in estrogen. First-phase BBT tends to stay low and is predominantly affected by estrogen activity. Second-phase BBT, which is noticeably higher, is controlled by progesterone. 

In the absence of pregnancy, basal body temperature usually dips approximately one to two days prior to menstruation. Of course, this may differ from one individual to the next, and it will also vary slightly from cycle to cycle.

How accurate is BBT-based ovulation tracking?

According to statistics, this form of birth control is 99-percent effective in an ideal setting. When circumstances are less than ideal, efficacy hovers around 75 percent. That’s why BBT charting should not be used as the only means of contraception.

Choosing a basal body temperature thermometer

Consider purchasing either an electronic or mercury thermometer for gauging basal body temperature. Then, make sure to use the same one throughout the entire measurement period.

Generally speaking, electronic thermometers are safer to use; they signal when peak temperature is reached and store all the information for future reference. In contrast, mercury thermometers must be shaken. (Try to do this the night before so the movements don’t affect your morning BBT.) Also, if a mercury thermometer reading falls between two markings, always choose the lower one.

Tracking BBT after a sleepless night

Struggling with insomnia? Avoid measuring basal body temperature until you’ve gotten at least three to four hours of continuous sleep.

If you work at night, check your BBT in the daytime after a few consecutive hours of restful slumber. Over time, the data on the BBT graph will prove to be far more reliable.

Detecting pregnancy on the BBT chart

If your basal body temperature remains elevated for more than 14 days after a leap indicating ovulation and your period doesn’t start, this could indicate pregnancy.

When conception has occurred, the fertilized egg implants into the endometrium 6 to 10 days later. This often translates to a steep decline in BBT, as the onset of pregnancy is associated with the release of estrogen. Subsequent progesterone production will soon cause it to rebound.

As a result, basal body temperature following conception may create a triphasic pattern on the BBT graph (i.e., a third temperature level appears).

However, these signs of early pregnancy aren’t infallible. To be on the safe side, consider taking a home pregnancy test or contacting your health care provider for confirmation.

When does BBT drop if you’re not pregnant?

If you didn't conceive during the current cycle, basal body temperature will dip again roughly 10 to 14 days after ovulation. This dip usually coincides with the start of your next period.

Flo user testimonial

One Flo user shares her story on how BBT-based tracking helped confirm she was pregnant:

What does a complete basal body temperature chart look like?

A typical BBT graph reveals day-to-day changes. A person’s menstrual cycle consists of two phases (occurring before and after ovulation). A normal basal body temperature chart is biphasic, resembling a bird flying upward. It’ll show the dips in temperature before ovulation and before menstruation (when conception does not occur). Immediately following ovulation, BBT rises and remains high until the premenstrual drop.

The BBT spike in the second phase should be at least 0.5 degrees Fahrenheit (0.3 degrees Celsius) higher than the first-phase BBT. Furthermore, it’ll last for about 10 to 14 days.

Next, let’s dive into the different types of basal body temperature charts and what they indicate.

Monotonous-curve BBT chart

When a BBT graph displays a steady pattern with no distinct highs or lows throughout the cycle, it could indicate the absence of ovulation. So, if you observe a low basal body temperature in the second phase, you probably didn’t ovulate at all.

This situation isn’t a cause for concern. In fact, some people experience several anovulatory cycles per year, which is completely normal. At other times, a faulty thermometer may be the culprit. Try again with a new thermometer and use an ovulation test. If nothing changes, consult your health care provider. 

Zigzag-curve BBT chart

When conception does not occur, the BBT chart should show two temperature levels. A low basal body temperature will appear during the first phase, and a high one will appear during the second.

A zigzag-shaped graph with constantly alternating low and high BBTs might point to an estrogen deficiency. This assumes the thermometer isn’t faulty and all logging instructions were followed correctly.

Think about adjusting your diet and paying closer attention to your overall physical and mental health, then continue monitoring your BBT for three more cycles. If you’re interested, speak to a gynecologist about what steps to take next.

Erratic-curve BBT chart

If a BBT graph shows a chaotic curve over several cycles, with dramatic temperature increases, it may also indicate a shortage of estrogen. (Dips and spikes in basal body temperature exceeding one full degree are considered dramatic.)

As always, external factors can interfere with BBT tracking results. They include a defective thermometer, inconsistent measuring techniques, or altered mental and physical states due to stress, fatigue, illness, etc. Make an appointment with your provider to follow up on these BBT results.

When is a BBT chart inaccurate?

Basal body temperature data is potentially incorrect when:

• It’s measured at different times of the day or with different approaches.
• You have a fever.
• You’re using certain medications.
• Alcohol was consumed the day before.
• Emotional or physical stress is present.
• The body’s circadian rhythm is disrupted (e.g., by jet lag).
• You sleep with an electric blanket, electric sheets, or a heating pad.
• You’re breastfeeding.
• You’re struggling with insomnia.

Charting BBT is not helpful when taking hormonal contraceptives because these synthetic hormones affect menstruation and ovulation.

By carefully following these guidelines, you’ll be well prepared to achieve the BBT chart results you want! The Flo app offers a great basal body temperature tracker, so be sure to download it today!