UNDERSTANDING MY MENSTRUAL CYCLE

In September 2020, I decided to try to quit hormonal birth control for the second time. Part of my plan included tracking my basal body temperature to know whether or not I was ovulating. Tracking temperature changes also allowed me to do a bit of detective work when my hormones were getting out of whack and delaying ovulation, and now it helps me to adjust my nutrition according to my menstrual cycle phase to regulate my blood sugar. It’s such a simple, yet powerful tool.

WHAT IS BASAL BODY TEMPERATURE (BBT)?

Your basal body temperature is the lowest body temperature your body experiences after a period of rest, and it is generally reached about at least 3 hours of uninterrupted sleep. In practical terms, your basal body temperature can be measured immediately upon waking after a normal night of sleep. It should be taken while still in bed, before you’ve started moving around, scrolling through your phone, etc. And it should be taken with a special thermometer called a basal body temperature thermometer, which includes 2 decimal places. It can be measured under your tongue, just like you’d normally take your temperature to check for a fever. Some wearable devices can also be used to automatically capture and record your basal body temperature. These include the Apple Watch and Oura Ring.

Basal body temperature changes alongside hormonal changes, which allows us to use it to track our menstrual cycle. Pretty cool, right?

MENSTRUAL CYCLE PHASES AND BASAL BODY TEMPERATURE 

Your menstrual cycle starts with the first day of your period. The menstrual phase lasts for the duration of your period. In my cycles in the graphic above, these are the days shaded in purple. If you are not on hormonal birth control, after your period, you enter the follicular phase, where your body prepares an egg for ovulation. In this phase, a follicle develops around the egg that will be ovulated. In the graphic above, my follicular phase includes the days after the purple shaded days and before the day marked with a little egg, which was the most likely day of ovulation. After ovulation, you enter the luteal phase of your cycle. During the luteal phase, the follicle that had enveloped the egg transitions into the corpus luteum which secretes progesterone. High progesterone levels cause an 0.5 to 1 degree increase in basal body temperature during the luteal phase relative to the follicular phase (Steward and Raja, 2022). You can thus detect ovulation by observing this increase in BBT on your BBT chart. 

In the graphs above, you can see higher temperatures, on average, after ovulation during the luteal phase. Your temperature will drop back down at the start of your period.

The basal body temperature method is a long-standing form of natural family planning, and I personally think it is the most accurate among various fertility awareness methods. 

IS BASAL BODY TEMPERATURE A PERFECT PREDICTOR OF OVULATION?

Sadly, not many things in life are perfect. A small amount of women do not experience an increase in their body's temperature with ovulation. A recent study using ultrasound to confirm ovulation noted that 98% of cycles had an associated increase in BBT, while 2% of confirmed ovulatory cycles did not (Ecochard et al. 2023). So if you have PCOS and you have a period without a preceding temperature rise, you may either be one of the few women who do not experience a temperature rise or you may be one of the many women with PCOS who have periods without experiencing ovulation (Burgers et al. 2010). In that case, cervical mucus monitoring might be a better method of cycle tracking for you, although I have found this to be less accurate for myself, or you can consult with your healthcare provider about their testing options.

You can also see from the graphs that BBT can have some random spikes and dips that have nothing to do with ovulation or hormones. A poor night of sleep, alcohol consumption, fighting off a cold, etc. can all result in extra temperature variation. However, a decent BBT tracking app can help you still interpret your temperature trends over the length of your cycle. These apps also learn your daily basal body temperature patterns and will flag and discard an abnormal temperature.  

CAN TRACKING BASAL BODY TEMPERATURE HELP ME GET PREGNANT?

Maybe… BBT tracking for fertility is a little tricky. The temperature rise occurs after ovulation and for some women, this rise occurs up to three days after ovulation, at which point, you have missed your fertile window. But tracking regularly can help you determine approximately when you are ovulating each cycle so that you can attempt to get pregnant in the fertile period prior to observing the increase in BBT. After a few cycles, you'll also have a good sense of when you ovulate to know when to use ovulation test strips. 

Can tracking BASAL BODY TEMPERATURE help me prevent pregnancy?

Potentially! Tracking BBT allows you to pretty accurately identify fertile days, and the Natural Cycles app is actually an FDA-approved form of birth control! On days that you are potentially fertile, you should always use some form of birth control which might include abstinence, using a male or female condom, using a diaphragm, etc. The Natural Cycles app flags days to use back-up protection for you. In the graphs above, these days are shown in red. Talk to your healthcare provider about your best birth control options!

BASAL BODY TEMPERATURE AND THE ARRIVAL OF YOUR PERIOD

Perhaps the most useful aspect of basal body temperature charting is knowing when your period is arriving. Irregular menstrual cycles can make any day potentially day 1 of your period, and that’s a scary situation to be in! With the BBT method, you have a pretty close guess of when you ovulated. For most women, the luteal phase lasts 12 to 14 days. While the follicular phase can vary widely from cycle to cycle for women with PCOS (as can be seen in my cycles depicted above), the luteal phase is pretty consistent, so after tracking a couple of cycles, you will have a pretty good idea of when your period will arrive after your detected ovulation. Your BBT also drops on the day your period arrives, which is a great warning sign to be prepared.

WHAT ELSE CAN BASAL BODY TEMPERATURE TELL ME?

Tracking BBT has been most helpful for me during cycles that ended up being longer than normal. My cycle is now about 30 - 35 days long. But when I first started cycling naturally, I had some substantially longer cycles. Without tracking, I would not have know that things were off track until day 35 or so. Now, if I hit day 20 or so with no sign of ovulation in sight, I know I need to make adjustments somewhere. Maybe I’ve been skimping on sleep, or maybe I have had higher levels of stress. Or maybe I’ve been going on one too many sugar benders. In any case, the lack of ovulation prompts me to check in and reevaluate what I’m doing, so that I can course correct earlier in my cycle. 

I now also use tracking to know exactly where I am in my cycle to adjust my nutrition. Through continuous glucose monitoring, I have learned that I am very insulin resistant for about a week before my period arrives (You can read more about that here.) By tracking BBT, I know when I’ve ovulated and when I’m entering this insulin-resistant phase so that I can adjust my carbohydrate intake accordingly. This has been a critical strategy for eliminating my last remaining PCOS symptoms.

WHAT’S THE BEST WAY TO TRACK BASAL BODY TEMPERATURE?

For me, using the Natural Cycles App has been really helpful. It has a great algorithm for predicting your ovulation, and it plots your data on cool graphs, like the ones included above. If you’re an Oura ring or Apple Watch user (I am not… yet anyways), you can sync these devices to the app, so that you don’t even need to remember to take your basal temperature in the morning! 

​Given the current political climate, some people don't feel great using period tracking apps, and I totally get that! You can make your own paper chart or you can log your data in an Excel file. To get you started, I've actually created an Excel file that will automatically graph your temperatures for you!

BASAL BODY TEMPERATURE TRACKING STEPS

1. Download a BBT tracking app like Natural Cycles App. If you subscribe with the link included here, you’ll get a BBT thermometer as well. 

2. If not using Natural Cycles or another app with a thermometer included, purchase a BBT thermometer.

3. Put your thermometer where you will see it first thing in the morning. When I travel, I use an elastic to secure my thermometer to my phone, which I use as my travel alarm clock. When I’m home, I have a sunrise alarm clock, and I place the thermometer in front of the clock. The light essentially spotlights the thermometer, so I don’t forget to use it.

4. Before getting out of bed, take your temperature and record your temperature readings.

5. Do this every day and wait until ovulation.

6. Once you’ve ovulated and your BBT rises, marvel at how your body works! And congratulate yourself on ovulating. It’s no small feat, especially if you have PCOS!

7. Keep on tracking, so that you keep getting to know your unique cycle.

Note, if you get to 15 or 16 days after ovulation, you may have a potential pregnancy. Go take a pregnancy test and/or see your health care provider. 

REFERENCES

Burgers, J.A., S.L. Fong, Y.V. Louwers, O. Valkenburg, F.H. de Jong, B.C.J.M Fauser, J.S.E. Laven. 2010. Oligoovulatory and Anovulatory Cycles in Women with Polycystic Ovary Syndrome (PCOS): What’s the Difference? The Journal of Clinical Endocrinology & Metabolism 95(12):E485–E489.

Ecochard, R., H. Boehringer, M. Rabilloud, and H. Marret. 2023. Chronological Aspects of Ultrasonic, Hormonal, and Other Indirect Indices of Ovulation. British Journal of Obstetrics and Gynecology 108(8):822-829.

Steward, K. and A. Raja. 2022. Physiology, Ovulation and Basal Body Temperature. StatPearls. Available: https://www.ncbi.nlm.nih.gov/books/NBK546686/