Basal Body Temperature 101 (How to Track Your Cycle)

The two most common questions I get about women’s health are with regards to contraception and its opposite, getting and staying pregnant. There are so many birth control options out there (with risks/benefits to each) and so many conflicting pieces of advice for women who are trying to become pregnant. Amid that sea of confusion, the number one tool I recommend to patients, friends, and family for both of these purposes (getting pregnant and avoiding pregnancy) is basal body temperature (BBT) charting.

*Note: in this post, I make a lot of references to hormones and phases of the menstrual cycle. If you are unfamiliar with these topics, I encourage you to first read these posts:

What is BBT and why should we track it?

Basal body temperature refers to the temperature of our bodies when we are at rest, typically measured after a period of sleep of at least four hours. When we’re awake, up and “at it” in our daily lives, our metabolisms are revved up and producing heat as a by-product from using energy. But when we’re sleeping, energy output is low, and we’re in an environment with a stable temperature, allowing our bodies to find an optimal hot/cold balance.

In addition to exercise, eating/drinking, and the tasks of life, our hormones have a large influence on the temperature of our bodies. However, unlike the activities of daily living, our hormones influence our body temperature while we are at rest. While thyroid hormones do play a role, their influence tends to be more steady/stable in comparison to sex hormones—namely estrogen and progesterone. Progesterone is especially responsible for changes in basal body temperature because it creates an increase of about 0.2-0.4 degrees Celsius when levels are at their highest versus when they’re at their lowest.

This temperature change from progesterone is important because once that rise first starts to take place, it means progesterone is being produced at a high level. As I describe in this post about female fertility, this progesterone increase is triggered by ovulation. By charting basal body temperature, we can learn the exact day on which we ovulate in a given cycle, predict the day of ovulation in a future cycle, and identify our fertile days, which is helpful both if we’re trying for a pregnancy or trying to avoid one.

How to Chart BBT

To track your basal body temperature, you need two things: a basal body temperature thermometer and a means for graphing your temperature points. Some women prefer to do this the old fashioned way with pen and paper, but I like to use an app, such as Fertility Friend or Kindara. Keep in mind that in order to accurately chart, you need a thermometer that reads up to two places past the decimal. Most oral thermometers only read to on place past the decimal, and these are not accurate enough for this purpose. BBT thermometers are affordable (usually less than $20) and you can find them on amazon. Here’s a link to the one I use.

Temperature readings need to be taken as close to resting as possible, which means first thing in the morning after you wake up from at least 4 hours of uninterrupted sleep. As much as possible, try not to roll or move around too much (or get up to use the bathroom) before taking your temperature. Keep your thermometer in a place that is easily accessible from your bed, like your night stand. Since I log my chart in a phone app, my morning starts off with waking up, taking my temperature, recording it in my app, and then doing everything else. It took a few days to get the hang of it, but now it’s just part of my automatic routine.

Interpreting BBT Charts

If you look back at the previous graphic, you’ll see that the very top line corresponds to body temperature. As already mentioned, progesterone is responsible for the cyclic increase in basal body temperature after ovulation, and decreasing progesterone levels correspond to the decline of basal body temperature back to the baseline low. This rise/fall of progesterone and subsequent rise/fall of BBT is the basis for charting.

Throughout the follicular phase, our BBT is at a low. The day after ovulation, it takes a sharp rise, usually of more than 0.4 degrees, due to the sharp rise in progesterone. This temperature rise is sustained until a few days before your period, when it slowly drops back down again. As you can see on this chart example, there is a notable increase in temperature from 97.7 degrees on cycle day 15 up to 98.3 degrees on cycle day 16. Based on this information, we can interpolate that ovulation took place on cycle day 15. (We know this from our understanding of hormones that the corpus luteum formed after ovulation produces progesterone.)

When interpreting your own BBT chart, look back over your first month of charting to see when the temperature rise first took place. The day before this temperature rise corresponds to the day of ovulation. This is the most important piece of information gained from tracking BBT.

What your chart can tell you

Charting empowers us to be able to identify the day of ovulation with a very high degree of accuracy. With the menstrual cycle, ovulation is everything, and it is what informs our knowledge of conception, contraception, and everything else. Knowing when we ovulate is key in terms of timing sex to promote chances of pregnancy, or avoiding unprotected sex in order to prevent pregnancy. Ovulation is the key factor in conceiving a child, and if the lifespan of the sperm doesn’t overlap with the day of ovulation, it’s impossible to get pregnant. Fertile days are calculated in relation to the day of ovulation, including the 5 days prior to, the day of, and the day after ovulation (7 days total). Outside of that “fertile window,” pregnancy is pretty much impossible. (As with any form of birth control, surprises happen. But with this highly accurate, informed method, they are rare.) Along the same lines, we know that the days with the best chance of conceiving are the day of ovulation and the two days prior. When you know what day you ovulate, you can time sex to optimize your chances of success.

  • When to expect Aunt Flo: In addition to knowing the number of days in your average cycle, tracking BBT gives clear information about when your next period is due. Just as rising temperatures signify elevated progesterone as the result of ovulation, falling temps towards the end of the cycle indicate that hormone levels are dropping, and your next period is just a few days away. If you have an “off cycle” due to stress or travel or something and your period is coming early or late, BBT will give indicators by falling temperature. I’ve found this to be helpful, as surprise visits are never welcome.
  • Conceiving a pregnancy: If you’re trying to get pregnant, charting BBT gives you the best information as to what your fertile days are. Look back on your last cycle and identify which day of your cycle you ovulation. For the next cycle, your best chances of conceiving will be the day of ovulation and the two days prior. While it’s a good idea to have sex at least every other day for about a week (starting the five days leading up to ovulation), you can focus your efforts on a few days to have better accuracy and feel less pressure.
  • Contraception: You can’t get pregnant if you don’t have sex during your fertile window. By using BBT to inform the days you do and don’t use protection, you can avoid the health risks and unpleasant side effects of products like birth control pills, implants, and IUDs. If you want to be ultra conservative, I recommend using a barrier method like condoms for the seven days of your fertile window plus three for a total of ten days. This would correspond to the seven days before ovulation, the day of ovulation, and the two days after. Of course, a barrier method is necessary to reduce risk of sexually transmitted infections, but if you’re in a relationship where pregnancy is your only concern, then a mixed method such as this one could be a good fit for you and your values.
  • Anovulation: In addition to telling us when we’re ovulating, BBT charting can help shed light on situations in which we aren’t ovulating. While not ovulating often results in a missed period, it’s possible to have periods (even regular ones) in which ovulation doesn’t take place. (This is what happens with birth control pills. They cause a monthly bleed via artificial estrogen but prevent ovulation.) If this is the case for you, it’ll show up on your chart. Instead of having a biphasic chart, which refers to the two periods of time before and after the temperature rise, your chart will be monophasic. There will be no distinct temperature rise.
  • Early pregnancy detection: Another phenomenon of BBT charting is early pregnancy detection. As you can see on the chart example above, body temperature starts to fall starting at around cycle day 28. This results from decreasing progesterone levels as the cycle comes to an end. In a non-pregnant cycle, the corpus luteum is at the end of its “life span” and has stopped producing hormone. The uterine lining is getting ready to shed. However, in a pregnant cycle, the embryo implants around 6-12 days after ovulation and starts producing estrogen and progesterone in addition to what the corpus luteum is making. Because of this, the progesterone levels stay high (and steadily increase) rather than tapering off. By the end of your cycle, if you’ve had unprotected sex during your fertile window, and your BBT remains high, there’s a good chance you could be pregnant. Additionally, some women experience what’s called an “implantation dip” around the time that the fertilized egg becomes embedded in the uterine lining. On a chart, this looks like a sudden drop in temperature of 0.3 degrees or more followed by a sharp return the following day. Another sign of pregnancy (though this is present in up to 5% of non-pregnant cycles too) is what’s called a triphasic chart. After implantation in some pregnant women, a third temperature rise takes place. Instead of a baseline, rise, and plateau in a biphasic chart, a triphasic chart will have a second rise and plateau. Keep in mind that implantation dips and triphasic chart appearances are not completely reliable methods for identifying pregnancy. The only way to know for sure that you are pregnant is to have a positive pregnancy test, which is most accurate on or after the day of your expected period. (As a side note…many women take tests early, but false negative rates are very high until the day Aunt Flo is due.)

Troubleshooting Your Chart

BBT charting can be confusing the first month you do it. I know for myself, I thought I was doing something wrong because my chart seemed like it was all over the place. However, by the time I made it to the end of the month, I could clearly see the expected pattern, and was relieved to know that I hadn’t messed it all up.

Because body temperature is sensitive to the environment, things like illness, alcohol, restless sleep, travel, stress, or excessively hot/cold sleeping temperatures can create fluctuations in temperature. Despite these outliers, the pattern is still readily apparent. Just keep in mind that you might need to make it through your first whole cycle before you get the full picture.

As an example, here is a picture of one of my first ever charts. I had a few days where the temperatures deviated quite a bit from my baseline, but the app I used (Kindara) allowed me to mark those days as “questionable” (seen as gray dots) which gave me a clearer overall picture.

On this chart, I included a horizontal line to show the threshold I measured between my follicular phase and my luteal phase. In this cycle (which is my norm) I ovulated on cycle day 14, which was followed by a sharp temperature rise from 97.3 degrees to 97.7 degrees on cycle day 15. I knew immediately that cycle day 15 was the first day of my temp rise because it was the highest temperature I had recorded the entire cycle except for day 2 when I knew the higher temperature was an outlier. During my follicular phase for this cycle, my average temperature was about 97.4 degrees and it was about 97.8/97.9 degrees during my luteal phase. (I had a few really high temperatures but they sharply dropped off right before my next period.)

On this chart, you can also see the orange line which represents “egg white” quality cervical mucus. I don’t typically chart my mucus but I do pay attention, especially on the days when I have watery or egg white cervical mucus, as these are high indicators of fertility. I have also used urine test strips to detect luteinizing hormone as a triple confirmation of when I’m ovulating. By now, I know for sure how to identify my fertile days through charting and observation.

In addition to temperature and cervical mucus, many charting apps include options for tracking other symptoms like breast tenderness, spotting, positive or negative pregnancy tests, positive or negative ovulation tests, and more. There are different free and paid apps out there with different capabilities, and I encourage you to try out a few to find one that works for you!


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