Basal body temperature: Detecting ovulation and early pregnancy

What is basal body temperature?

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 GynecologistsOpens a new window (ACOG), in most women, this temperature increases slightly (0.5 to 1.0 degrees Fahrenheit) during ovulation.

If you're trying to get pregnant, you can track your basal body temperature to estimate when you'll ovulate and determine the best days to have sex (or be inseminated). For greater accuracy, you can combine tracking your BBT with monitoring changes in your cervical mucus.

It's important to note that your most fertile period is in the two to three days before the increase in BBT. So when you detect a temperature change, your optimal window for getting pregnant will likely have already passed.

"BBT alone is not accurate," says Sasha Hakman, M.D.Opens a new window, M.S., an ob-gyn and reproductive endocrinologist in Los Angeles and member of the BabyCenter Medical Advisory Board. "It only tells you once you've already ovulated, which makes it too late to time sex or avoid sex to conceive or avoid conception."

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:

1. Menstruation. During your period, your BBT will likely be between 97 and 98 degrees F.
2. 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).
3. 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.
4. 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.

To get your BBT, take your temperature when you first wake up in the morning – before you eat, drink, have sex, or even sit up in bed or put a foot on the floor. The Mayo ClinicOpens a new window says to make sure you get at least three hours of uninterrupted sleep to ensure an accurate reading. Try to take a reading at about the same time each morning, and record it on a BBT chart (see below). If you don't take your temperature immediately after waking up, your BBT chart won't be accurate.

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

Our BBT chartOpens a new window gives you a handy way to track your basal body temperature. You can also use it to track your cervical mucus.

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:

1. 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.
2. 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.
3. 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
4. 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.
5. 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.
6. 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 chartOpens a new window.

When you look at the sample chart, remember that every woman's cycle is different, and your personal chart may not look like the example or even be the same month to month. You may also want to try one of the apps available for charting BBT (and other ovulation symptoms).

Look for a pattern in your temperature rather than one single spike. Overall, your BBT will likely be lower before you ovulate than after you ovulate. Some women notice a temperature dip on the day of ovulation, before the rise.

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.
• Then you may notice a few days of cloudy, sticky discharge.
• In the few days leading up to ovulation, the amount of discharge increases and becomes thin, slippery, and stretchy (like egg whites). This consistency makes it easier for sperm to travel through the cervix to the egg. These are your most fertile days.
• Just after ovulation, the amount of mucus decreases and becomes thicker.
• Then you may be dry for several days before your next period.

A good time to check your cervical mucus is when you go to the bathroom first thing in the morning, but you can check it any time of day. Sometimes you may be able to see cervical mucus on the toilet paper after you wipe. Other times you may need to insert a clean finger into your vagina (toward your cervix) to get enough mucus to examine.

"When using methods that rely on cervical mucus, be aware of any changes in your health or daily routine that could make reading the signs of ovulation difficult," advises ACOG. "Medications, feminine hygiene products, douching, sexual intercourse, breastfeeding, or having a pelvic exam in which lubrication is used all can change how the cervical mucus appears."

Look for other signs of ovulation

There are other signs that you may be ovulating, too. Your cervix will feel softer and be higher, wetter, and more open than usual. You may feel ovulation cramping on one side of your abdomen, or you may have light spotting or bleeding when your body releases an egg.

Hormones may cause you to retain fluid and feel bloated and your breasts to feel tender. You may notice an increased sex drive. Some women even say they have a heightened sense of smell and a smaller appetite when they're ovulating.

Use an ovulation predictor kit

An ovulation test strip (or ovulation predictor kit, OPK) can measure your hormone levels and predict when you're going to ovulate in the next 24 to 36 hours (a good time to have intercourse if you're trying to get pregnant).

There are urine-based OPKs that test for an increase in luteinizing hormone (LH), which typically rises within a day and a half before ovulation. And there are salivary ferning kits, which predict ovulation by examining the salt content of your saliva, which changes as your estrogen levels rise in the days before ovulation.

You can buy OPKs online or at the pharmacy (usually near the pregnancy tests).

Use our Ovulation Calculator

If you know the first day of your last period and the length of your cycle, you can use our ovulation calculator to predict your ovulation date and find the days you're most likely to be fertile.

Your fertile window includes your ovulation day and the five days before ovulation. You're most likely to get pregnant in the last three days of this timeframe.

Learn more:

• How to boost your chances of getting pregnant
• The most common early signs of pregnancy
• How and what to eat if you want to get pregnant