A basal body temperature (BBT) chart is quite simply a graph showing your basal body temperature (your temperature at the very moment you wake up in the morning after at least 4 hours of sleep) throughout the month. It shows slight differences in a temperature and as such gives a snapshot of menstrual hormonal health.
It is done for a variety of reasons. It is just one aspect of the Fertility Awareness Method (FAM), and can be used to avoid pregnancy or to enhance chances of conception. The idea is that if one knows their fertile window, they can optimize it and/ or keep sperm away during that time, depending on what your goals are at this time in your life. In my practice I consult my patients’ BBT charts for the purposes of enhancing fertility, and this is where I get the most questions. This blog post will be most helpful to those of you trying to conceive.
It can come as a shock after years of scrupulously avoiding pregnancy to discover that it does not happen as easily as one might have been led to think in high school. (Although to be fair, the youthfulness of high school bodies may make conception a lot easier, so good that you were so careful.). After a few months of failed attempts, couples often begin to wonder if they should take a closer look at what they are doing. Conception in the best of situations can take many months; it is good to shift lifestyle habits towards optimal health and begin gentle investigation early while also resting in the knowledge that bodies and nature do things in their own time. There is a delicate balance to be struck between optimizing your odds and being relaxed.
A BBT chart can confirm something very important — whether or not the ovaries are ovulating. The only other way to know this is by carefully tracking with imaging what the ovaries are up to throughout the cycle. “But wait!” You say, “my OPK strips tell me that I am ovulating!” Actually, they measure your luteinizing hormone (LH). This is your body’s messenger that tells the growing egg it is time to pop out of the ovary. Your body can send this message, but the ovaries do not always respond. A BBT chart is a great way to see if the eggs are leaving the ovary, and also to give a general picture of hormonal health.
BBT charts, like egg white cervical mucus (EWCM) and ovulation strips, can also tell a couple when the sperm should be trying to meet the sperm. Whether this is through intercourse or through the insertion of donated semen, the timing is important. BBT charts can only tell you you have ovulated after the fact (more on this below), but after a few months of tracking you can come to see when your fertile window falls. I personally do not think this is the best way to time sperm meets egg, as the EWCM is much more important for timing – the fertile mucus both keeps the sperm safe and helps it to get where it’s going at the right time. (Read more about this here.).
Finally, one more use of a BBT chart is to be able to see changes in hormonal health. A BBT chart is absolutely unnecessary to know if hormonal health is improving. There are always other indicators to track, such as quality of sleep, abundance of EWCM, bowels, urination, and most obviously, menstrual symptoms. However, it can be fun and satisfying for the patient (and your herbalist and acupuncturist, if you have one) to see the chart become more and more textbook each month. This is not for everyone. Some people find it fun, some find it stressful, so once you have confirmed that indeed you do ovulate, do as you please.
Okay, I’m ready to try it, how do I do it?
Go to the pharmacy and get yourself a thermometer with two decimal points. The tenths of a degree is where you will see the temperature fluctuation. The moment you wake up, before you speak, get up, go pee, sip water, stick the thermometer in your mouth and let it sit there a few minutes so that it is your temperature. Then push the button, wait for it to beep, read the temperature and record it. You can record it in an online fertility tracker or on a good old fashioned spreadsheet. (If you like the paper versions I have one free for download here.) Typically these spreadsheets and apps also have a line to track cervical mucus (CM), when there was sex or insemination, and other data. (This is highly personal data…this data will also show pregnancies and miscarriages. If you prefer not to share this with a third party, use Google to find which apps are available currently that commit to storing your data locally only.) All of these signs together show a complete picture of your fertile window.
But what am I looking for?
Okay, so you’ve done the work, you have a cool graph, but how do you interpret it? You want to see a “biphasic” chart, which means a clear delineation from a more or less steady line at a lower temperature from the first day of your period (cycle day one) to midcycle, when you should see a clear and distinct stair step of half a degree. After the temperature rises, it should stay up until right before you start bleeding. It is the hormone progesterone that increases your body temperature just after ovulation, and it is the withdrawal of this hormone that causes the uterine lining to shed. If right before your period your temperature does not drop, but instead goes up and stays up another half a degree, this is called a triphasic chart. Triphasic charts can often indicate pregnancy!
Let’s take a look at a real live charts pulled from public forums on the internet to see what this can look like and how it might change.
The first one is example of why it is helpful to temp. The LH surge is occurring (the red line) but the blue line never goes up and stays up about half a degree.
