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BBT Basics - real talk about basal body temperature tracking



What is Basal Body Temperature tracking?


Basal Body Temperature tracking is the practice of taking your temperature (orally or at the armpit - axillary) first thing in the morning before you get out of bed. Your basal temperature fluctuates throughout the menstrual cycle and can be used to retrospectively confirm ovulation, as well as assess general hormone function. Estrogen is the hormone that rises as our ovaries are preparing eggs for potential ovulation, and progesterone is produced after ovulation. Estrogen lowers our body’s metabolism, so we tend to see relatively lower ranges of temperatures prior to ovulation when estrogen is rising. On the other hand, progesterone increases our body’s metabolism so we observe relatively higher temperatures after ovulation. This is what allows us to confirm ovulation retrospectively - once we see at least 3 temperatures that are all 0.2 degrees (fahrenheit) higher than the previous 6 temperatures, we can confirm ovulation.



What can be assessed through BBT tracking?


BBT tracking allows you to both confirm ovulation, as well as assess for anovulation. Anovulation is where you have a period or bleed, then your body might try to ovulate -but it ultimately doesn’t, and then you get another bleed. The bleed after anovulation is called a “withdrawal bleed”. Beyond ovulation, you can assess relative thyroid function and stress response through overall low basal body temperatures or irregular/”jumpy” temperatures, respectively. This data point is not used to diagnose thyroid dysfunction, but if you are experiencing symptoms of hypothyroidism and are also noticing generally low temps, this could be an indication to meet with your practitioner to discuss whether or not thyroid testing might be appropriate. Additionally, if you were to notice very irregular or “jumpy” temps and you were to implement strategies to reduce your stress levels, assessing your BBT can be a great tool to determine whether or not these strategies are effective.



Types of BBT shift


There are a few different types of BBT shifts, and they can all be considered within the range of normal. The first is an abrupt or acute shift. This is where the BBT rises sharply between one day and the next. The acute BBT shift levels off at its highest point within a 48-hour period and accounts for the majority of BBT shifts. The second type of shift is the slow shift. The BBT rises gradually, taking 5 or more days to reach its highest level. The third is a step shift. This is where the BBT rises slightly to a plateau and it remains for at least 2 days, then rises again to a new plateau, continuing this pattern for 5 or more days until it reaches its highest level. The step-like BBT shift is much less common than the previous two. And the final type of BBT shift is the saw tooth pattern. This type of BBT shift rises through peaks and valleys. This pattern is much more rare and can be difficult to interpret. It also requires a longer observation before you’re able to confirm ovulation.


Regardless of the type of BBT shift, when you are confirming ovulation you want to draw a baseline/coverline. Different sympto-thermal Fertility Awareness Based Methods have different rules about how to draw a coverline, but a coverline is a horizontal line that is drawn after confirming ovulation to differentiate between the preovulatory and post-ovulatory temperatures. Ideally, all of our post-ovulatory temperatures will remain higher than the baseline until the day before or the first day of our period.



Where does ovulation happen in comparison to the BBT rise?


Much like there are variations of normal in regards to the type of BBT shift, there is also a statistical range where we expect ovulation to occur based on the initial rise. Most ovulation occurs the day before the temperatures cross the baseline (29%), but ovulation can occur anywhere from 3 days prior to the initial shift (2%) and up to 2 days after the shift (11%). The second and third most common days of ovulation in relationship to the BBT shift are the day after the shift (27%) and two days before the shift (24%) (Gnoth et al, 1996).




How does BBT tracking help to prevent unintended pregnancies?


While tracking your mucus and assessing Luteinizing Hormone levels can help to determine which days you’re likely fertile, tracking your BBT allows you to confirm ovulation, which indicates the beginning of the infertile period. Once the ovulatory window closes you are no longer at risk of ovulating and are therefore considered infertile until the beginning of the next cycle. If you were to only use BBT as a contraceptive method, you would have to consider yourself fertile from the beginning of your menstrual phase until 3 days after your BBT rise. However, there are many ways to assess the fertile window and a sympto-thermal method of Fertility Awareness would use at least one marker to open the fertile window (typically this is based on in-real-time observations of cervical mucus or sensation changes, or it can include a standard calculation based on your earliest signs of fertility), as well as a retrospective assessment of ovulation through a BBT shift. Observing that BBT shift and giving it at least 3 days to confirm, allows you to confidently enter your infertile phase.



Does BBT tracking help when trying to conceive?


Yes….and, no. While tracking your BBT does not give you any insight into when your fertile window opens (therefore when to strategically time intercourse), if you are tracking your cycle for several months you’ll be able to assess some patterns that your body might be showing you. For instance, you might notice that you tend to see your thermal shift 2 days after your Peak Day of mucus, or the same day as your LH surge. This can give you insight into where ovulation may be happening in relationship to these other biomarkers, so you could have a bit of a head’s up in the scenario that your body continues to follow that pattern. Additionally, it’s helpful to track your BBT for a few cycles before trying to conceive so that you are able to confirm regular ovulation, as well as assess for any luteal phase dysfunctions that may indicate a hormone imbalance (which would be important to address before actively trying to conceive).



Best practice when tracking your BBT


While the process of BBT tracking is very simple and straightforward, the results are not always the case, especially when your sleep is disrupted. Generally speaking, it’s best to take your temperature within an hour or so range on either end. So if you typically wake up at 7am during the week, you can take your waking temperature anywhere between 6-8am. If there are days of the week that you wake up more than an hour earlier or later than your average waking time, you’ll still want to take your temperature when you first wake up, but just note that there was a disturbance to your temping time on your chart. This way, if this wider window results in inaccurate temps, you’ll be able to see the irregularity later on when you’re looking at all of your temperatures - and if you know it was on a day with a disturbance you can discard that particular data point. Obviously, if this happens with regularity you’ll end up with too few data points to actually see a trend. Additionally, if your sleep itself is irregular (staying up quite late some nights and therefore getting significantly less sleep overall, or having interrupted sleep - hello early years of parenthood, etc.) that can also create some challenges with observing a clear trend. I wouldn’t let these scenarios prevent you from beginning to integrate BBT tracking for a few months, as you may be surprised by the data you’re able to collect! As a birth doula, I often have disrupted sleep (or go 24+ hours without sleep). But I continue my tracking process, just noting the days where my sleep patterns were disturbed. When I have months where I’m attending lots of births (or during the timeframe where I used to do a lot of overnight postpartum care), my temps are often quite wonky. And I usually see evidence of continued stress in my body (and therefore on my chart) for a day or two after I pull those all-nighters. As I’ve always said, the charts don’t lie.

It’s not just about the data that we’re able to collect and apply for fertility purposes, but the information we’re able to receive from our bodies regarding patterns and sources of stressors so that we can make strategic adjustments to support our overall health and wellbeing.

Tips to improve BBT tracking


One of the tricks I’ve used to ensure consistent BBT tracking is placing my thermometer on top of my phone before going to bed. Since I use an alarm to wake up in the morning, I have to physically touch my thermometer before turning off the alarm. This prompts me to take my temperature, and I even use a preset snooze on my phone to allow my thermometer to warm up in my mouth (while I’ve fallen back asleep) for 10 minutes before taking my temperature with the second alarm. I have an easy time falling back asleep in the morning, so I actually set my alarm for 6:50am to prompt me to put my thermometer in my mouth, a second alarm at 7am to then take my (glass non-mercury) thermometer out of my mouth after it has assessed my temperature, and then a third alarm for when I actually have to get up in the morning. I do this every day to keep my temping process consistent, but I understand that this doesn’t work for everyone. If you tend to wake up with a lot of energy, you could implement the pre-warming process (holding the thermometer in your mouth or under your armpit for 5-10 minutes before assessing your temperature, which helps to increase the reading’s accuracy) while doing your morning “scroll”, checking your email, or - better yet - implementing a meditation/breathwork/gratitude practice before getting out of bed.


I also encourage you to plan to track your temp regardless of your wake times. As mentioned earlier, if there are days of the week where you sleep in much later than normal, just continue to temp and then note that the timeframe that you took your temperature was abnormal (and therefore the data may be disturbed). The more consistent you can be, even if things aren’t exactly the same each morning, the better….as the more data you’re able to collect, the more likely you’ll be able to observe a thermal shift.


Just like the changes in sleep times, if there are some nights with more restless sleep, more waking times, or if you end up getting up to pee less than 3 hours before your normal waking time, you’ll still want to temp as usual. You can just note that there were some disturbances to your sleep on your chart and then if those readings are wildly different than the other data points you collect that cycle, you’ll know that you can discard and ignore those readings.


And if after reading all of this you think to yourself, “There’s no way I’m going to remember to take my temperature first thing when I wake up.” Or, “My sleep is way too disturbed to track my BBT”, you can consider using a wearable BBT thermometer like the TempDrop. The TempDrop is an armband that you put on at night and it takes your axillary (underarm) temperature throughout the night to assess a relative trend. At this time I don’t necessarily encourage people to use the Ava bracelet, Oura ring, or Apple watch for BBT, only because the research on BBT tracking at these parts of the body is not yet robust. But with more time there may be enough research to indicate that these are appropriate places to assess temperatures and they may be reliable enough for contraceptive Fertility Awareness purposes. And after supporting tons of folks who have used a TempDrop over the years, I got one for myself! I’m still in the midst of comparing its results to my waking oral temperatures, but once I have a few cycles worth of data, I’ll be sure to update this post with my findings. If you’re interested in using a TempDrop, you can use this link to get 10% off!


Curious about integrating BBT into your cycle tracking process? Questions about what you’re noticing? Book a session with me to dive further into this topic (and many, many more)!


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