What does my temperature have to do with my menstrual cycle?
At the beginning of the 20th century, Dutch gynaecologist Theodoor Hendrik van de Velde first described the relationship between basal body temperature and the menstrual cycle (1). He noticed that there was an upward shift in basal body temperature (also known as BBT) following ovulation which lasted until the onset of menstruation. Based on these findings daily temperature tracking was later developed as a fertility awareness method used by women to help estimate the fertile window.
Temperature tracking can be a time consuming process and is easily influenced by external factors, some of which we will discuss a little later. We now know however that the shift in body temperature observed across the menstrual cycle is actually a response to varying levels of progesterone (2,3) but how does progesterone drive these changes and can progesterone tracking be used as an alternative method to estimate the fertile window?
Temperature tracking is an indirect way to monitor your progesterone levels to identify when ovulation has taken place. Hormone tracking allows you to measure these changes directly.
How does progesterone affect my body temperature?
Progesterone is a hormone released by the corpus luteum in the ovary (4). It plays an important role in the menstrual cycle and in maintaining the early stages of pregnancy. You can read more about the roles of progesterone in our article Progesterone 101. During the first half of your menstrual cycle, your progesterone levels are low (4). As a result your BBT is slightly lower during this phase of the cycle. After ovulation, the corpus luteum, which is the structure leftover after ovulation, begins to produce progesterone. This marks the onset of the luteal phase. At this stage, progesterone is required to prepare the uterus for implantation of the fertilized egg. The rise in progesterone causes a slight increase in your body temperature acting via the hypothalamus, which is the part of the brain that regulates our internal body temperature (2,3). Exactly why the rise in temperature is necessary remains unknown; however, some research suggests that it may help with the implantation process of the fertilized egg3.
Throughout the rest of your cycle, your progesterone levels and basal body temperature remain relatively high. If no conception occurs the corpus luteum breaks down, causing progesterone levels and BBT to fall and menstruation to begin. The shift in body temperature over the menstrual cycle can therefore be used as a way to follow progesterone production and identify when ovulation has taken place.
Temperature tracking requires discipline
While temperature tracking sounds simple enough, daily charting of BBT can be a time consuming process and is often prone to human error.
Your BBT is your body’s lowest resting temperature. For the most accurate results temperature measurements must be made at the same time every day. In fact it is recommended that measurements are made as soon as you wake up, even before sitting upright in bed.
Another important thing to note is the rise in BBT after ovulation is very slight - from 0.5ºF/0.3ºC to 1.0°F/0.6ºC3 - and can only be detected using a special type of thermometer with 0.1ºF/0.01ºC accuracy1. BBT can be easily influenced by other factors such as fever, alcohol consumption, emotional stress, change of room temperature or even changes in sleep (1,3).
What makes hormone tracking so convenient
We now know that temperature tracking is an indirect way to monitor your progesterone levels to identify when ovulation has taken place. Hormone tracking allows you to measure these changes directly.
Through a straightforward saliva test the inne mini lab monitors daily changes in your progesterone levels. By building a baseline during the follicular phase, the system can then detect the increase in progesterone levels during the second half. This will help identify the onset of your luteal phase and confirm ovulation has taken place. With this data inne is able to predict when your fertile window will occur in your cycle.
Unlike temperature tracking, which must be measured at the same time every day, with inne you simply pick a four hour time window - anytime between 5 am and 2 pm - to take your daily saliva sample. All inne requires is that you don’t put anything in your mouth in the 30 minutes before you take your sample.
Are progesterone levels in saliva reliable?
While hormone levels such as progesterone are conventionally measured in blood plasma, studies have shown that saliva measurements can be an alternative way of tracking hormone changes across the cycle (5,6). Progesterone levels in saliva have been shown to follow the same patterns measured in blood plasma - a rise after ovulation, followed by a smooth and broad luteal peak (4,5,6).
As well as being less invasive and allowing for easy testing at home, saliva measurements may also give a more informative reading of hormone levels. The majority of hormones exist in the body as one of two forms, free or protein bound. Free hormones are able to enter cells which means they are biologically active. In contrast protein bound hormones are inactive as they are unable to pass through the cell membrane and instead remain in the blood circulation. When blood is filtered through the salivary glands only the unbound free hormones are thought to pass through into the saliva. As a result saliva testing measures the free biologically active and clinically relevant form of the hormone (7).
1. Su, H-W., Yi, Y-C., Wei, T-Y., Chang, T-C., Cheng., C-M. Detection of ovulation, a review of currently available methods. 2017, Bioeng Transl Med. 2, 238 - 246.
2. Steward, K, Raja, A. Physiology, Ovulation And Basal Body Temperature. In StatPearls (Updated 2020). Available from: https://bit.ly/3lEmiW0
3. Charkoudian, N., Stachenfeld, S. Sex hormone effects on autonomic mechanisms of thermoregulation, 2016, in humans. Auton Neurosci. 196, 75 - 80 .
4. Reed, B. G. & Carr, B. R. The normal menstrual cycle and the control of ovulation. In Endotext (eds. De Groot, L. J., Chrousos, G. & Dungan, K., 2015)
5. Lu, Y-C., Bentley, G. R., Gann, P. H., Hodges, K. R. & Chatterton, R. T. Salivary estradiol and progesterone levels in conception and nonconception cycles in women: evaluation of a new assay for salivary estradiol. 1999, Fertil Steril. 71, 863 - 868.
6. Gann, P. H., Giovanazzi, S., Horn, L. V., Branning, A. & Chatterton, R. T. Saliva as a Medium for Investigating Intra- and Interindividual Differences in Sex Hormone Levels, 2001, in Premenopausal Women. Cancer Epidemiol Biomarkers Prev. 10, 59 - 64.
7. Hofman, L. F. Human Saliva as a Diagnostic Specimen., Nutr. 131, 2001, 1621S - 5S. Available from: https://bit.ly/3oo1yDt