The Most Accurate Way to Pinpoint the Fertile Window and Ovulation

Groundbreaking Study Reveals most Accurate Way to Pinpoint the Fertile Window and Ovulation

Research shows that no matter how fertile you are, if you have sex outside of your fertile window, you most likely will not get pregnant (1). In fact, you may have as low as a 0% chance of getting pregnant before your fertile window and only one day after ovulation. If you are trying to conceive and not yet pregnant, you may be wondering how to pinpoint your fertile window and how to find out if you are even ovulating.

While there are a variety of methods to try and pinpoint your fertile window, many have caveats. For instance, hormone-detecting urine sticks (ovulation prediction kits) don’t work as well as they claim on the package (hard to interpret, some women do not have hormone levels high enough to be detected and they don’t work for women with irregular periods) and require peeing on a stick each morning and afternoon for a duration of time. Morning basal body temperature (BBT) charting is time consuming and the results are typically inaccurate due to all the variables that affect oral temperature readings, such as medication, restless sleep and even having a glass of wine the night before. Saliva tests have generally been found to be inaccurate with little science behind them (2).  

Currently, the recommendation to pinpoint your fertile window is to use a combination of both BBT and another method, such as ovulation prediction kits. Tracking BBT alone does not predict the fertile window: temperature rises once a woman has ovulated, which is typically too late for conception. Tracking BBT can help to determine if you are 1) ovulating 2) help predict your ovulation cycle for the next month and 3) track for any potential ovulation issues. Daily temperature readings, however, are a lot of work, and studies have shown mixed results for daily BBT readings to be a reliable method for determining ovulation (3). The question then becomes: is temperature really an accurate method, and why are studies showing mixed results?

What scientists noticed, is that a couple of days before ovulation, women have a subtle dip in temperature. For over 30 years, a nadir (aka dip) in temperature has been identified as a potentially useful biomarker to predict ovulation (4). Studies following this prediction, however, were unable to consistently find the nadir in temperature among all the women participating in the studies (Barron et al 2005). This is because these studies used once-per-day oral BBT measurements. That is until Mary Coyne and colleagues used current technology to measure women’s CONTINUOUS and CORE body temperature (6). Their results were groundbreaking and shattered previous studies’ conclusions that temperature is too variable to predict the fertile window.

Watch Here to Learn about Priya for Fertility

The previous studies used a single measurement of oral or rectal temperature readings at fixed times in the morning, which is not an effective measurement to identify changes in circadian rhythm. Coyne and colleagues had women swallow an internal temperature sensor which measured core temperature readings throughout the day. They found unequivocal data results demonstrating that women have a significant decrease in their core temperature 1-2 days BEFORE their urinary LH-surge. The study results showed that if there was an internal sensor that women could use to measure their core temperature throughout the day, their fertile window would be precisely and effortlessly found each month.

Wouldn’t it be amazing if there was such a product like this that existed? A way to effortlessly and accurately measure core temperature that doesn’t disrupt normal daily life? We thought so. That is why Prima-Temp, a biotech company that measures circadian rhythm has developed a vaginal temperature sensor, the Priya Ring. A woman can insert the flexible ring (comparable to the NuvaRing but without the hormones) once a month and go about life as usual. When you are most fertile, the wearable sensor sends a message to your phone. You will be able to view your circadian rhythm data to see precisely when you are most fertile, and also to see if you had a rise in temperature to indicate that you have ovulated. Since the Priya Ring is inserted once a month and measures core temperature all day and night, it eliminates the possibility for human error and is the most accurate reading possible. Priya Ring will even work for women with regular and irregular cycles, as well as for women with PCOS.  


Priya is truly effortless, precise, and empowering.



  1. Wilcox, A. J., Dunson, D., & Baird, D. D. (2000). The timing of the “fertile window” in the menstrual cycle: Day specific estimates from a prospective study. British Medical Journal, 32, 1259-1262. 
  2. Fehring RJ, Gaska N. Evaluation of the Lady Free Biotester in determining the fertile period. Contraception 1998;57:325–
  3. Barron ML, Fehring RJ. Basal body temperature assessment: is it useful to couples seeking pregnancy? MCN Am J Matern Child Nurs 2005;30(5):290-296; quiz 297. L. E., Lee, S. G., Levy, W., Woodruff, J. D., Wu, C. H., & Abdalla, M. (1974). The ovulatory cycle. A histologic, thermal, steroid, and gonadotropin correlation. Obstetrics & Gynecology, 44, 14-25. 
  4. Coyne et al. (2000). Circadian Rhythm Changes in Core Temperature over the Menstrual Cycle: Method for Noninvasive Monitoring Physiol Regulatory Integrative Com Physiol. AM J 279:1316-132