The science of the menstrual cycle is amazing, and harnessing that science is how people have used the Symptothermal Method to successfully avoid pregnancy for decades. If you want to know a little more about it, read Phases of the Menstrual Cycle, but I’ll briefly talk about some terms and how ovulation and temperature are related.
Important: The information below is not meant to be used as a guide for avoiding or achieving pregnancy. This is for informational purposes only. Please research the Symptothermal Method thoroughly and meet with a qualified coach to learn the Symptothermal Method.
Basal body temperature (BBT) can be used to confirm ovulation, but the common misconception is that we take our temperature in the morning to measure basal body temperature. That’s not what’s happening. Basal body temperature is defined as the lowest temperature your body reaches in a 24-hour period — that usually happens between 3am and 6am (1, 2). Sure, some of us may be regularly awake at 6am, but that doesn’t mean our body temperature was at its lowest point at that time on that day. What we’re actually measuring when we wake up is waking temperature. It’s not the same as basal body temperature but is generally a close enough approximation to get the information we need to confirm ovulation.
The menstrual cycle is the follicular phase, ovulation, and the luteal phase. The follicular phase begins with the first day of your period and ends on the day of ovulation. During this phase, BBT and waking temperature are generally lower in this phase. The luteal phase starts the day after ovulation and ends the day before your next period. The day after ovulation, BBT and waking temperature shift to higher levels (compared to the follicular phase) for the duration of the luteal phase. This pattern with low temps in the follicular phase and high temps in the luteal phase is called a biphasic pattern (3).
When charting oral temperature manually, you look for the sudden shift from low to high temperatures to confirm ovulation. This is referred to as the temperature shift (or temp shift), and it indicates ovulation happened the day before the shift. These higher temperatures must be sustained for at least 3 days before ovulation can be confirmed (3).
It sounds reasonable enough, except that waking temperatures can be finicky and noisy. That is, it’s not always clear if the temperature is valid because they can by affected by many variables, including but not limited to (3):
- How long you slept
- How well you’ve slept
- Certain medications
- Drinking alcohol the day before
- Getting up during the night
- Waking up earlier or later than normal
These are just a few factors that cause unwanted “noise” in a chart that makes temping confusing and why charting can be difficult for people in general, but maybe even impossible for people who work the night shift or regularly get up in the middle of the night to pee or for caretaking responsibilities.
The bottom line is that taking oral temperatures in the morning can be useful, but it’s not perfect.
- Centers for Disease Control and Prevention. (2014, April). Providing Quality Family Planning Services Recommendations of CDC and the U.S. Office of Population Affairs, Appendix D: Contraceptive Effectiveness.
- B S, H., K, D., R C, M., T G, K., & A, P. (2022). Novel Technique for Confirmation of the Day of Ovulation and Prediction of Ovulation in Subsequent Cycles Using a Skin-Worn Sensor in a Population With Ovulatory Dysfunction: A Side-by-Side Comparison With Existing Basal Body Temperature Algorithm and Vaginal Core Body Temperature Algorithm. Frontiers in bioengineering and biotechnology, 10, 807139.
- Weschler, T. (2015). Taking Charge of Your Fertility: The definitive Guide to Natural Birth Control, Pregnancy Achievement, and Reproductive Health.