Your basal body temperature (BBT) is your lowest body temperature in a 24-hour period. It's the temperature of your body when you're at rest. According to the American College of Obstetricians and Gynecologists (ACOG), in most women, this temperature increases slightly (0.5 to 1.0 degrees Fahrenheit) during ovulation. 

What is basal body temperature?

Tracking your basal body temperature might help you predict when you'll ovulate and identify the ideal times for intercourse (or insemination) if you're trying to get pregnant. You can track changes in your cervical mucus and your BBT together for increased accuracy.

It is noteworthy that the two to three days preceding the rise in BBT correspond to your most productive time. Therefore, it's possible that your ideal window for getting pregnant has already passed when you notice a change in temperature.

If you track your BBT for a few months, though, you can identify patterns that will help you predict ovulation. (More on how to do this below.)

Your BBT chart can also give you a sign that you're pregnant. If you track your BBT and notice that the rise in your basal body temperature after ovulation lasts beyond when you'd normally get your period, you may be pregnant. (Of course, not getting your period is another sign of pregnancy!) This would be a good time to take a pregnancy test.

There are other reasons women track their BBT:

If you're having problems with fertility, tracking ovulation can be very helpful.

If you're using the rhythm method to prevent pregnancy, you can use your BBT chart (in addition to other tracking methods) to guess which days to avoid unprotected sex.

BBT and your menstrual cycle

Your menstrual cycle begins on the first day of your period and typically lasts from 21 to 35 days, with an average of 28 days. Your BBT will vary according to which phase you're in.

Here's what you can expect:

Menstruation. During your period, your BBT will likely be between 97 and 98 degrees F.

Follicular phase. During this phase (which starts on the first day of your period and lasts until the first day of ovulation) BBT usually stays between 97 and 98 degrees F. About one day before ovulation, BBT typically dips about 0.5 degrees to its lowest point (on average, 97 to 97.5 degrees F).

Ovulation. Once you ovulate – around day 14 of your menstrual cycle – progesterone production causes a rise above your baseline BBT of 0.5 to 1.0 degrees F.

Luteal phase. BBT stays high throughout much of this phase (sometimes called the "two week wait"). If you're not pregnant, your BBT returns to its lower range about a day or two before you get your period.

Does BBT detect early pregnancy?

It may.

When you ovulate, your temperature rises slightly, thanks to the production of progesterone. If you're not pregnant, your body will stop producing progesterone right before your period starts, and your temperature will drop back down to your normal basal body temperature. If you're pregnant, your body will continue to produce progesterone, and your temp will remain elevated.

Sometimes – if a woman's body produces even more progesterone now that she's pregnant – the BBT will rise even a bit more rather than drop.

So, if your luteal phase is a day or more longer than it usually is (you see a continued higher BBT) it may mean that you're pregnant. You'll also be missing your period at about the same time, though, so your BBT won't be your strongest pregnancy hint!

What's a normal basal body temperature?

Before ovulation, your BBT may range from about 97 to 98 degrees F (97.2 to 97.7 degrees F, to be more exact). But the day after you ovulate, you should see an uptick of 0.5 to 1.0 degree in your BBT, which should last until about your next period.

You may notice your temperature occasionally spiking on other days, but if it doesn't stay up, you probably haven't ovulated yet.

In addition to ovulation, the following can affect your BBT:

• Pregnancy
• Fever
• Some medications, such as antibiotics, blood pressure medicines, and birth control pills
• Some diseases, such as thyroid disorders
• Some gynecological disorders, such as endometriosis
• Exertion
• Hot weather
• Physical or emotional stress
• Interrupted sleep cycles, getting too little or too much sleep
• Alcohol
• Travel and time zone differences
• Shift work, such as working the evening shift
• Room temperature. (Try to keep a consistent temperature in your bedroom overnight.)

How to take your basal body temperature

To get an accurate reading, use a thermometer that's accurate to 1/10th of a degree, which is sensitive enough to measure tiny changes in body temperature. Special basal thermometers are sold in pharmacies and online for this purpose. Although some digital ones give readings to a hundredth of a degree, all you really need is one that will give a reading to one-tenth of a degree. Thermometers that only give readings to two-tenths of a degree aren't accurate enough.

Take your temperature as soon as you wake up in the morning, before you eat, drink, have sex, sit up in bed, or even step onto the floor, in order to determine your BBT. To achieve an accurate reading, the Mayo Clinic advises getting at least three hours of unbroken sleep each night. Make an effort to take a reading every morning at roughly the same time and record it on a BBT chart (see below). Your BBT chart won't be accurate if you wait to take your temperature right after waking up.

You can start charting your temperature anytime, though the first day of your menstrual period is a good time to start. Continue charting through your cycle and you'll have a complete picture at the end.

How to use a BBT chart

Print out some copies of our blank chart, buy a basal thermometer, and you're ready to start charting. Here's how to do it:

On the first day you get your period, fill in the date and day of the week under cycle day 1. Continue noting the dates of your cycle until the first day of your next period.

Each morning when you wake up, immediately take your temperature with a basal thermometer. Put a dot next to the temperature that matches your thermometer reading for that day. (You can also note the time you took your temperature.) Connect the dots to see how your basal temperature fluctuates from day to day.

You can also check your cervical mucus each day if you wish. Record the type of discharge you find each day, according to the key at the bottom of the chart: P = period, D = dry, S = sticky, E = egg-white-like

Toward the end of your cycle, watch for a day when your BBT rose 0.5 to 1 degree F and stayed high. That day is usually the day you ovulated. It should correspond with the last day you noticed egg-white-like cervical mucus.

Track these symptoms for a few months to see if you notice an uptick in BBT and egg-white-like mucus at the same time each cycle. That will allow you to plan which days to have sex if you want to get pregnant.

For the best chance of conceiving, have sex at least every other day during your most fertile period.

If you want to see what a chart looks like when it's completed, take a look at our filled-in sample chart.

Keep in mind that every woman has a unique cycle, so even if the sample chart may seem the same, your own chart may fluctuate from month to month. Moreover, you might want to give one of the apps for tracking BBT (and other ovulation symptoms) a try.

Instead of focusing on a single spike in your temperature, look for patterns. In general, your BBT will probably be lower prior to ovulation than it will be afterwards. On the day of ovulation, some women experience a drop in body temperature prior to an increase.

Other ways to track ovulation

To increase your accuracy, combine BBT tracking with other methods of identifying ovulation.

Examine your cervical mucus

Cervical mucus is vaginal discharge produced by the cervix. Over the course of your menstrual cycle, the amount, color, and texture of your cervical mucus changes due to fluctuating hormone levels.

Checking your cervical mucus and keeping track of these changes can help you tell when you're most fertile. It's a good thing to track in tandem with your basal body temperature. Here's what to watch for:

Once your period stops, you may not have any discharge for a few days.

After that, you can experience a few days of sticky, hazy discharge.

The amount of discharge increases and gets thinner, slicker, and more elastic (like egg whites) in the few days before ovulation. Sperm find it easier to pass through the cervix and reach the egg because of this consistency. You are most fertile right now.

Just after ovulation, the amount of mucus decreases and becomes thicker.

The days leading up to your next menstruation may then be dry for you.

Although you can check your cervical mucus at any time of day, it's best to do it right after your morning restroom break. Occasionally, after wiping, cervical mucous may be seen on the toilet paper. In some cases, you might need to get enough mucus to examine by sticking a clean finger into your vagina (in the direction of your cervix).

Look for other signs of ovulation

There are more indications that you might be ovulating. In addition to feeling softer, your cervix will be higher, wetter, and more open than usual. When your body releases an egg, you can experience light spotting or bleeding on one side of your abdomen during ovulation.

Hormones can make your breasts feel painful and lead you to feel bloated and retain fluid. There can be a noticeable rise in sex desire. Some women even claim that when they are ovulating, their sense of smell is enhanced and their hunger decreases.

Use an ovulation predictor kit

Your hormone levels can be measured with an ovulation test strip, also known as an ovulation predictor kit, OPK, which can also estimate when you will ovulate in the next 24 to 36 hours—a favorable time to have sex if you're trying to get pregnant.

A surge in luteinizing hormone (LH), which usually occurs a day and a half before ovulation, can be detected by certain urine-based OPKs. Additionally, there are tests called salivary ferning that use an analysis of your saliva's salt content—which varies when estrogen levels rise in the days preceding ovulation—to forecast when a woman will ovulate.

Online or in the drugstore (typically next to the pregnancy tests), OPKs are available for purchase.