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Why You Shouldn't Trust Fertility App Predictions (& Instead Learn to Chart Yourself!)

“What app do you recommend for charting?” 

It is one of the most common questions I get when I teach an NFP introductory session. My answer is pretty non-committal. Some women like to sync their FitBit or Apple Watch; others want to chart moods. One woman wants to share her chart with her spouse, and another really likes the color purple.

Every woman has her own priorities. Regardless of which app they choose, I always give this caveat: do not go by the app’s fertile window.

Personally, I have tried 12 apps over the last few months. Initially it was an attempt to discover which one might be the golden standard that I would recommend to my clients.

Then I just got curious. How did they program their algorithms? Some apps asked if I was breastfeeding (I was) yet still gave the exact same fertile window as another that did not ask for those details.

My conclusion: Your app is a tool, a convenient way to chart. Not a substitute for learning a method of fertility awareness. 

Natural Family Planning (NFP) and Fertility Awareness Method (FAM) advocates have worked long and hard to dispel the myth that it is simply “the rhythm method” repackaged. When using a true method that monitors legit biomarkers, NFP is scientific and effective even with all of the irregularities of life. An app’s predicted fertile window is exactly what we’ve been trying to avoid: the return of the calendar method. 

The calendar methodMost apps are set to predict your next month’s ovulation based on the length of your previous cycle. Once you enter multiple periods, the app will count back 14 days from your current period and consider that the “peak” day from your last cycle. Whatever cycle day that was, it will anticipate a similar date for your next cycle, add 5-6 days before that, and give you an estimated fertile window. It is exactly that: an estimate based on a calendar

Hormones are the driving force behind fertility. Estrogen dominates the beginning of the cycle, LH shows up half way through, and progesterone wraps it all up before the next period. These hormones are particularly sensitive to stress, life changes, medication, other hormones (i.e. thyroid), etc. They are not predictable based on a calendar. 

If you are in the first 6 cycles breastfeeding, your ovulation will move earlier with each cycle. If you are getting married this month or Christmas with your family is particularly stressful, your ovulation may be delayed by a week or more. A bout of the flu could throw off your peak day due to the stress on the body. A calendar calculation is not able to account for situations such as these which alter the days of fertility. 

Understanding a temperature shift, monitoring cervical mucus, or testing urinary hormones for the presence of estrogen or LH are all ways to monitor fertility with accuracy during the craziness of daily life. Cervical mucus and the presence of estrogen in the urine metabolites both indicate the body is preparing for ovulation and fertility is high. A peak hormone monitor reading (indicating the presence of LH) or a temperature shift both confirm ovulation has occurred. 

To be clear, an app designed to interpret a temperature shift or adjust based on recorded cervical mucus observations could be accurate. However, this can never be a substitute for understanding the method yourself and being able to double-check the app’s findings. 

If ovulation is approaching earlier in one cycle, the app’s predicted fertile window may not account for those days. Presence of cervical mucus or a reading on a fertility monitor can alert you to the anticipated earlier ovulation in a way that the app simply cannot.

If ovulation is delayed, waiting for confirmation of ovulation is important for those trying to avoid pregnancy. Progesterone confirms ovulation, and the presence of progesterone in the body causes the body temperature to rise. A woman may think she is beyond her fertile window based on her app’s prediction but has not yet seen a temperature rise. This is a red flag to watch for a delayed ovulation because her body’s biomarkers have not indicated ovulation yet. 

The primary biomarker for my clients is urinary hormones. #NFPeeonALLthesticks is how I like to describe it. 

However, mucus and temperature observations are also options for my clients. One client recently had a bout of severe seasonal allergies and was on medication that dried up her mucus. The stress on her body delayed her ovulation by a few days, pushing the anticipated fertile window identified by her app. I counseled her to wait and watch for her temperature shift. Thanks to her temperature readings, her ovulation was confirmed with accuracy, four days later than she expected it to come.

Apps are very convenient. Our phones rarely leave our person, so charting is accessible and easy to reference. When an app is used as a record-keeper, it is a great tool for fertility awareness. In order to accurately determine your fertility status throughout the month, however, you need more information than an app can gather. Fertility hormone and temperature monitoring are accurate, objective markers of fertility. Learn how to use these from a qualified instructor, and you can determine when you’re fertile regardless of the stress and circumstances life throws at you.  

Rebekah Knobeloch Marquette NFP InstructorRebekah Knobeloch is registered nurse and fertility educator in the Marquette Model of Natural Fertility Planning (NFP).  She blogs at Feminine Genius Ministries and can be reached at RebekahKnobeloch@MarquetteNFP.com.