Your Basal Body Temperature Chart: What to Know and How to Read it (properly)

I think I first heard of basal body temperature charts when I was a teenager and I read “Women’s Bodies, Women’s Wisdom”, by Christiane Northrup. It didn’t mean much to me then, but over the years I’ve kept running into it. When I recently started seeing an acupuncturist, she gave me a basal body temperature chart to fill out as part of my file.

I dutifully obliged, but found all those numbers very perplexing, though she seemed to get a lot out of it. It really spiked my curiosity about what kind of valuable information I might be missing out on by not understanding what all of that means. So over the next few weeks I researched all I could find on the subject and became fascinated.

I’ve continued to keep a record of my basal body temperature on and off just to keep an eye on things, but I find it also helps me to keep in touch with my body.

I know that there are many women out there who are also using this method for fertility awareness. I thought many of them might find this information useful when starting out, since it can feel a bit confusing at first.

Below I will give you the basics on how to interpret your own basal body temperature chart for fertility awareness. I will also point out a few red flags to watch out for.

I decided to include information on how to check your cervical mucus as well, since it is such a valuable tool to combine with basal body temperature and they do tend to go hand-in-hand.

Table of Contents

What is basal body temperature

Basal body temperature (BBT) is literally your base body temperature (at rest).

A woman’s basal body temperature will change throughout her cycle, as a result of changing hormone concentrations in the body.

The average body temperature will vary between individuals, and depending on the time of the day and the activity. Temperatures typically range from about 97°F (36.1°C) to 99°F (37.2°C).

For many women it can be a useful tool for assessing fertility, whether you’re trying to get pregnant or to prevent it.

What affects your BBT

Besides hormones, there are several factors that will influence your basal body temperature and, therefore, your basal body temperature chart pattern. Among them are:

illness
sleep disturbance, change in waking hour, and shift work
emotional or physical stress, including traveling (specially changing time zones)
alcohol intake
fasting
change of room temperature or change of climate
certain types of medication, such as the start or discontinuation of birth control pills or antipyretics (fever medication)

How charting your BBT can help you

You can use your basal body temperature chart for:

Fertility awareness: it can be very useful when trying to determine when you ovulate, so you are able to better plan sex for conception, or to prevent unwanted pregnancies.
To help detect fertility problems
Help detect pregnancy at an early stage.
Signal underlying health conditions: it can be a useful indicator of other health issues, such as a thyroid condition.

How to chart your BBT

This is what you need to do to create your own basal body temperature chart.

You start on the very first day of your period.
First thing in morning, before moving AT ALL (no sitting up or rolling over as much as possible), slip the thermometer into your mouth. Actually, you can use it orally, vaginally or rectally, just make sure to use the same method every single time.
Try to wake up at roughly the same time every morning, within a window of no more than 30 minutes.
Mark the time and temperature on a chart.
Make sure to note any unreliable days: illness, less than 4 hours of uninterrupted sleep, jet lag, medication, fasting days, heavy drinking, etc.
You can also note any cramping, spotting, or type of cervical mucus – all these are possible indicators for ovulation and it might help you recognize it on a chart.
Do this for at least 3 months and try to detect a pattern.

What thermometer to use

Ideally you would use a thermometer that can measure up to 2 decimal points, so as to be able to detect more subtlety in the temperature changes and be able to round up or down.

Any BBT thermometer on the market will do this and there are many to choose from. Nevertheless, a plain ol’ regular glass thermometer that is accurate to 1/10^th of a degree will suffice.

Remember a basal body temperature chart has been a popular family planning method since the Thirties (not really known for its digital tech), so it’s totally up to you.

One benefit of digital thermometers is that it’s definitely less of a hassle than trying to suss out where the end of the line is on a traditional thermometer, when you’re still half asleep and zombified.

Having said that, I personally use a traditional glass thermometer, both for low waste Eco reasons and because of a personal esthetic preference. It works fine for me – it’s totally up to you.

How to interpret a basal body temperature chart

The pattern of temperatures will look slightly different for every woman.

At least 3 higher than average temperatures in a row is probably a sign that ovulation occurred, most likely the day before the first high temperature. Your best bet to get pregnant would be to make sure to have sex on the 2 days prior to the first day of this temperature rise.
There might be a sharp temperature dip on the day of ovulation itself, just before the temperature rises. If you see it tends to happen every month, it could be a very strong sign of ovulation occurring and, therefore, a very good day to have sex, if you’re trying to get pregnant.

Another possible indicator of ovulation is something called mittelschmerz. These are lower belly cramps, resembling menstrual cramps, that may be concentrated on one side only. It may be accompanied by some spotting.

Dividing the menstrual cycle into two halves, before ovulation and after ovulation, you should notice a different trend overall, on your basal body temperature chart:

Before ovulation, a woman’s BBT average is slightly lower, between 97°F (36.1°C) and 97.5°F (36.4°C)
After ovulation, it can rise to 98.6°F (37°C)

Basal body temperature usually only rises by about 0.5ºF (0.3ºC) to 1.0°F (0.6ºC). This rise in temperature is caused by the rise of progesterone following ovulation that lasts throughout your luteal phase (second half of the cycle).

At the end of the phase, if no pregnancy occurs, the progesterone levels will drop, triggering your period and a temperature drop.

Here you can find several examples of basal body temperature charts with corresponding interpretation.

If you feel a bit lost, try and find the one that most resembles your chart, for some direction. You can also take a look at all the different patterns in order to get better acquainted with the type of thing you should be looking for in yours.

Looking at the whole cycle, you will notice little spikes here and there, but try to see if you can notice an overall pattern.

As mentioned before, typically you would see a biphasic (2 phase) pattern – overall lower before ovulation, overall higher after ovulation.

If your cycle isn’t very regular, or you are having a difficult time establishing when ovulation occurs, avoid unprotected sex between the start of menstruation and until three days after the temperature rise. (*)

How to know if pregnant

On your basal body temperature chart:

If you see that the temperature stays high for 18 days or longer after ovulation (longer than usual luteal phase), that could mean that your progesterone has not yet dropped and therefore you might be pregnant.
A second temperature rise about one week after ovulation (triphasic – 3 phase – pattern) on your basal body temperature chart might also be an indicator.
Light temperature drop about 1 week after ovulation could be what’s known as “implantation drop”, although this one is not a very reliable indicator.

Related content: 14 Proven Benefits of Sex & Orgasm for Women

Basal Body Temperature Chart Red Flags

1 – Anovulation (lack of ovulation)

Irregular cycles, or cycles that are too short or too long (shorter than 25 days; longer than 31-33)
A luteal phase (second half of the cycle) shorter than 9 days
A lack of a sustained rise in temperature over those 3 or so days
A lack of fertile cervical mucus (see further below)

2 – Thyroid condition

Thyroid hormones have a stimulating effect on thermogenesis (production of heat) and energy balance.

Due to the involvement of these hormones in body temperature regulation, thyroid conditions are known to affect a person’s sensitivity to heat or cold.

Healthy thyroid levels are also an important factor for ovulation and a healthy normal length luteal phase.

A lower than average BBT can indicate hypothyroidism
A higher than average BBT can indicate hyperthyroidism. Excessive sweating is often a common sign of it.

Body temperature is only one of possible indicators and should always be contrasted with other known symptoms of these conditions.

BBT as a natural family planning method

The best way to use a basal body temperature chart for fertility awareness is to pair it with checking your cervical mucus. This is known as the symptothermal method. It can be highly effective, with an unwanted pregnancy rate of only about 1.8%. (*)

Cervical mucus varies throughout the cycle:

outside the period surrounding your ovulation, the mucus is thick, scarce, and viscous. This provides a barrier to pathogens and sperm.
during the period surrounding ovulation, due to the increase in estrogen, the mucus is more plentiful and resembles raw egg whites. This is highly receptive to sperm.

How to check your cervical mucus

There are three different ways of checking your cervical mucus:

Take a close look at discharges on your underwear
Wipe the opening of your vagina with toilet paper
Insert your finger into your vagina and check the mucus on your fingers

What does cervical mucus look like

Throughout your cycle you will have dry days, sticky days, and slippery days. All of these besides your period.

Right after your period there are usually a few dry days, meaning no mucus or discharge
During your follicular phase (first half of your cycle), as the follicles carrying different eggs start to ripen and prepare to release, the body starts to produce more mucus. It may be yellow, white or cloudy, and it should feel sticky.
Around the time of ovulation is when you’ll feel the most mucus. The consistency is that of raw egg whites and if you rub it between your fingers, it should feel stretchy. These are your slippery days (in every sense of the word) and they usually last about 4 days.
After that and right into your luteal phase (second half of your cycle), you will start to feel a lot less mucus. It should resemble the mucus of your follicular phase – sticky and/or cloudy. These will be followed by a few more dry days.

Your period should start after that and the cycle will repeat.

What affects the cervical mucus

Your cervical mucus can be affected by many different things, so it’s important to keep that in mind when checking:

Breastfeeding
Surgery on the reproductive system
Douching
Early menopause
Oral contraceptives, including the morning-after pill
STDs
Vaginitis
Having sex

Pregnancy – safe and unsafe days

Generally speaking, safe days are when you can have unprotected vaginal sex, without much risk of getting pregnant. These are:

After the most slippery mucus goes away and discharge is sticky again
Dry days of your luteal phase (second half of cycle) are the safest
For women with long cycles, the dry days after your period may also be safe

Unsafe days are when you’re most fertile and have a higher probability of getting pregnant. These are:

2 or 3 days before the first signs of slippery mucus, until about 3 days after slippery mucus peaks
The days during your period are also unsafe, especially for women who have a short cycle

A basal body temperature chart on its own, I would say, is most valuable when trying to get pregnant. This is not only because it can make you more intuitively aware of the best days in which to have sex with your partner, but also because even if you happen to do it on a non fertile day – what’s the harm? More fun for you!

However, if you want to use it as natural birth control, it would be wise to pair it with the information from your cervical mucus discharge.

A basal body temperature chart alone is very unreliable at avoiding unwanted pregnancies due to all the factors that can affect your body temperature. But paired with cervical mucus they are highly effective! They do require consistency and patience, so you really have to be motivated.

It’s definitely not for everyone, but if you feel inclined to try it, it can definitely help you read your own code. It’s like the cycle-syncing method – it makes you feel more in control of your body and more in tune with it.

Is anyone out there experimenting with a basal body temperature chart? What are your thoughts?