Why Guess When You Can Know? Continuous Core Temperature Predicts The Fertile Window

We ladies go to great lengths to find when we are fertile. With the possibility of conception only being 5-6 days a month, we use products to capture our daily basal body temperature readings (or BBT), hormones in urine or saliva, or have ultrasound appointments in an attempt to determine the elusive impending ovulation. Each method presents different disadvantages including high levels of inaccuracy, difficulty with irregular cycles, or in the case of ultrasounds, incredibly time-consuming and expensive.

In fact, research shows that women using ovulation prediction kits (detecting urinary hormones) to determine fertility only captured 30% of their fertile window (source). When using the calendar method, only a mere 3% of women with regular cycles were in their fertile window (source). This is a serious problem. Not only because daily intercourse (or intercourse every other day) throughout the 6 days of the fertile window significantly increase the odds of conception, but because many of us are starting our families at an older age and time is not on our side.

With technology advancing rapidly, is it possible to find a way to easily and accurately pinpoint our fertile window to increase our odds of pregnancy?

In the 1940s a number of fertility researchers from around the world, including a team with the World Health Organization, collected BBT charts and analyzed the data. The researchers developed sophisticated methods that required complex mathematics and charts to interpret BBT graphs. While they found their method accurately showed the day of ovulation, the math behind finding ovulation was found too complex for couples practicing natural family planning on their own (source).

While it certainty is nice to be reassured that our bodies are working and ovulation is happening, BBT only rises after ovulation has occurred. What is the percent chance of getting pregnant after ovulation has occurred? Zero (source).


priya-wilcox-graph .jpg

Women who use BBT often use it to estimate their fertile window for the next cycle.  The problem with estimating the day we ovulate is that ovulation varies each month, even if we have a regular cycle. Depending on the cycle length,  one study found women to ovulate anywhere between day 6 to day 60 of their cycle (source). This means that any method used to ‘estimate’ impending ovulation, for example the calendar method, apps and BBT, leave room to entirely miss the days we are most fertile.

The other methods used to prospectively mark ovulation, such as external temperature sensors, urine luteinizing hormone (LH) and salivary ferning kits, are found to be either inaccurate, cumbersome to use, do not work for women with irregular cycles, or in the case of LH kits, only capture a small part of the window such as 12 hours before ovulation, which causes a significant portion of women to miss their highest probably of conception.

When trying to get pregnant, what we really need to know are the exact 1-3 days right before we ovulate based on our own individual cycle. In 2000, researcher Mary Coyne and team conducted a study that measured core temperatures every minute over the course of an entire menstrual cycle using ingestible temperature sensors. These sensors were used in conjunction with fast and accurate cosinor analysis (aka pattern and rhythm analysis). The results found that by measuring continuous core temperature, oscillating temperature patterns can be identified to precisely pinpoint ovulation, and predict the critical preovulatory period, aka the fertile window.

While Coyne and team identified continuous core temperature as a biological cue to accurately predict the fertile window, how are we to utilize this method outside the research setting? Well, that is what we are working on. Here at Prima-Temp we are working hard to bring fertility to the 21st century for every woman whether they have regular or irregular cycles. We are making it possible to easily and accurately pinpoint the fertile window by taking advantage of advanced technologies in temperature sensing, algorithm development, and machine learning.


Soon it will be possible for you to easily and precisely predict your fertile window.


How do we do this? We have created Priya, a comfortable, effortless vaginal sensor that continuously monitors our core body temperature. You simply insert Priya at home when your cycle ends each month. Once inserted, Priya uses an algorithm to detect the subtle changes that occur PRIOR to ovulation to predict your fertile window. This allows for a complete picture of your cycle. The Priya app will automatically send an alert to your phone when you are most fertile.  

We have been testing Priya in clinical studies and are excited to share our data: Priya can predict ovulation 1-3 days before LH peak!  In other words, the temperature signals coming from continuous core temperature can predict the beginning of the fertile window, days before ovulation occurs. As mentioned above, these 1-3 days PRIOR to ovulation have the highest likelihood of getting pregnant.


Priya Clinical Data

Priya Clinical Data


The naked eye alone cannot always see the temperature pattern shifts, but Priya’s algorithm is able to find the subtle changes in temperature patterns that occur 1-3 days prior to ovulation (see graph above). The grey line indicates when ovulation is identified using a leading LH urine test strip, and the turquoise line indicates when Priya has identified the subtle temperature changes that occur prior to ovulation, when an alert is sent to your smartphone. By collecting core body temperature measurements continuously, and at the body’s core, we are able to track this signal in a way that was never before possible.

Daily activities such as exercising, bathing in a hot tub, drinking alcohol, and other activities that show fluctuations in temperature, making it difficult to find the ‘true’ temperature signal, are identified and extracted via the Priya algorithm and achieve a highly robust and accurate prediction algorithm. By wirelessly capturing continuous core temperature and eliminating potential for human error, Priya provides a level of precision that no other ovulation prediction method can.


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