Why Your Fertility Tracker App May Be Inaccurate


I am a woman, but I am also a scientist, which means I love anything that can help make my life as a woman easier, but I want to be sure the data it is accurate and reproducible. Therefore I’ve been doing an experiment recently to compare the effectiveness and predictive ability of several fertility tracker methods, including Basal Body Temperature charting (I’ll explain this in more detail below) and several common apps. 

Here’s how fertility tracking apps work… and why they may not be accurate for predicting your fertility

Fertility trackers allow you to input information about your cycle, such as the number of days of bleeding and the length of your cycle (from Day 1 to the next Day 1).  Based on these data points they predict your fertile window (the days when you are most likely to conceive).  If you are trying to get pregnant these are the days to have unprotected sex.  If you are trying not to get pregnant these are days to abstain from sex or use other contraceptive methods.  

Here’s the issue:

Many fertility trackers assume a typical 28 day cycle with ovulation on Day 14.  The major issue here is that there is great variety in women’s cycles both among women and within a single woman’s cycle.  Some women have 40 day cycles with ovulation on Day 20 or a 22 day cycle with ovulation on day 9, 10, or 11.  This has huge implications for your likelihood of getting pregnant each month.  For example, let’s say you have a cycle that is longer than 28 day- let’s say 35 days.  You are likely to ovulate closer to Day 18 or 19 but your app indicates you will ovulate on Day 14.  If you are depending on the fertility tracking app to tell you when you are most fertile and act accordingly you might be having unprotected sex during a phase of your cycle when you have slim to no chance of conceiving and you are completely missing your fertile window, which is likely Day 16-20.  In my practice, this accounts for a significant number of couples who are have difficulty conceiving simply because they are misinformed about when they are actually fertile.   

The good news is that our smart technology is smart.  Many of the fertility apps adjust the predicted ovulation date once you’ve been inputting your own data for a few cycles, to more accurately reflect your cycle.  But like I said, even a woman with the most regular cycle may have irregularities from time to time.  Travel, high stress, changes in diet, or a bout of food poisoning or the flu can all impact your cycle, potentially delaying ovulation.  

Let’s take another example:

Kate knows her body well.  She has been tracking her cycle consistently for several months and has observed a very regular pattern of a 30 day cycle with ovulation occurring on Day 15.  On Day 13 she comes down with a cold, complete with fever, chills, body aches and a stuffy nose.  Because she is also tracking her basal body temperature (her waking temperature) she sees that she didn’t experience a thermal shift (a temperature change from lower temperatures before ovulation to higher temperatures after ovulation, indicating ovulation), as expected on Day 15.  A week later, on Day 22 she observes a thermal shift, indicating ovulation has occurred.  Due to her cold she ovulated 7 days later than in her typical cycles.  If she were trying to conceive using the app, she would have completely missed her fertile window. This example is not uncommon.  I point this out because though your fertility tracker may indicate a fertile window based on your prior cycles, it cannot anticipate or predict circumstances like this.  

In my personal and professional experience by far the most accurate fertility tracking method is manual, pencil and paper, basal body temperature charting- or an app that allows you to input this data.  

Basal Body Temperature Charting is a fertility tracking method in which you take your temperature every morning as soon as you wake.  In the beginning of your cycle, from Day 1 of your period to ovulation your temperatures are lower, typically in the 97.0-97.7 range.  From ovulation to your next period temperatures rise in response to progesterone which is released to prepare your body for implantation.  During this phase temperatures typically rise to the range of 97.7-99.0 degrees. 

However, even this method is only effectively predictive after charting for several months, at which point trends begin to emerge. If after 2 months of charting you see that your cycle is typically 30 days and you see a thermal shift in your basal body temperature around Day 15, then you can anticipate the following month you will ovulate around Day 14 or 15 with an expected fertile window of Day 12-Day 15 or 16.  But what it also offers that other, more generic apps don’t is the ability to see when your temperature changes.  If your temperature hasn’t risen it is likely an indication that you have not yet ovulated, as in the case of Kate above. 

One more major fertility sign worth mentioning.  

Whether you are using an app or pencil and paper charting, one of the most valuable fertility signs that will help you know when you are most fertile is the presence of fertile cervical fluid (maybe the term “cervical fluid” grosses you out but this stuff is magic!).  This is a unique quality lubricative fluid, often described as egg-white-like cervical fluid that facilitates the movement of sperm.  When this quality is absent the sperm cannot survive or move effectively, disabling them and rendering them ineffective for conception.  Only when the egg-white-like lubricative cervical fluid is present can sperm make their way to an egg.  The quality of cervical fluid is perhaps the most important single predictor of impending ovulation and optimal fertility.  One way the apps can be useful is as an indicator of potential times to be aware of your cervical fluid.  On the flip side, if you’re wondering if the app’s predicted window is accurate, check for cervical fluid.  If it is present, it is a good indication you are indeed fertile and should act accordingly.  

To sum it up, if you are interested in using an app to help you get pregnant or prevent pregnancy look for one with the following features:

  1. Basal body temperature charting 
  2. Cervical fluid charting 
  3. Ability to record other relevant symptoms (i.e. cervical position, PMS symptoms, sex, etc.)

I recommend:

  1. Glow 
  2. Kindara

If you are interested in more guidance or have follow up questions don't hesitate to reach me at ali@yintuitionwellness.com or yintuitionwellness.com/book-now-

Happy charting!