The 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 a 0% chance of getting pregnant outside of your fertile window.

While there are a variety of methods to try and pinpoint your fertile window, you may have to explore several different options to find the one that works best for you.  

Hormone-detecting urine sticks (ovulation prediction kits) may work well for women with regular cycles but not for women with irregular cycles.  

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. If you have a solid sleep/wake routine and love charts, however, BBT charting may be a good way for you to gather information about your cycle. If you decide to use BBT, know that BBT alone does not predict the fertile window: temperature rises once a woman has ovulated, which is typically too late for conception (3).

Saliva tests have generally been found to be inaccurate (2).  


So which is currently the most accurate way to pinpoint ovulation?

With what is currently available on the market, you are best off doing a combination of methods - that is as long as they don't add more stress to your life. Doing a combination of BBT charting and the cervical mucus method can be quite accurate in estimating your fertile window. 

What about the new temperature sensors coming to the market? 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).

Temperature, therefore, got a bad reputation for predicting ovulation. This is because these studies used once-per-day oral BBT measurements. This all changed however, when Mary Coyne and colleagues used current technology to measure women’s continuous core body temperature (6). Their results were groundbreaking and shattered previous studies’ conclusions that temperature is too variable to predict the fertile window.


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  measure core temperature that doesn’t disrupt normal daily life? We thought so. That is why Prima-Temp, a biotech company that measures continuous temperature is working on developing a vaginal temperature sensor, the Priya Ring.  

Priya is currently being investigated to work as an internal sensor that sends a message to your phone when the algorithm detects the temperature changes indicating that you are about to ovulate. Check out our webpage to learn more about our progress.  




  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