Why You Shouldn’t Trust Apps that Predict Fertility

wrote this on July 18, 2014

There are thousands of apps in the world of period and ovulation trackers, and many of them claim to predict the days you’re going to be fertile. Wouldn’t it be great to have a crystal ball that would tell you with 100% certainty exactly when in the future you can and can’t get pregnant? You better believe I would be first in line for that one. Unfortunately, that’s not the way our bodies work. There are SO many internal and external factors that can have an affect on your cycle, so putting your trust in an app that claims to predict when you’ll be fertile isn’t the wisest choice (it’s essentially the same as relying on the rhythm method). In this post, I’m going to discuss just a few of the many things that can go wrong with fertility predictions and why you shouldn’t rely on these types of apps…especially to avoid pregnancy.

Would you trust this app?! Neither would I.

Would you trust this app?! Neither would I.

Delays in ovulation

Ovulation can be unexpectedly delayed by many, many factors, which can have a huge effect on when you’re fertile. Things like stress at work, stress in a relationship, moving, traveling, the flu, a cold, allergies, diet, and even heavy exercise can potentially delay ovulation. So that’s basically anything that puts stress on your body. When ovulation is delayed, your fertile window (or the time in your cycle when you can get pregnant) is effectively extended. Unfortunately, apps that predict fertility based on past cycle data – which is really any app that predicts fertility – cannot account for unexpected cycle changes that haven’t happened yet. Unless, of course, they’ve discovered a way to see into the future.

Fertile window length

Another point of failure for fertility predictions is the variable length of your fertile window. This can result from delays in ovulation, but that’s not always the cause. Let’s say that during your previous four cycles, you started producing cervical fluid on day 10 (or 10 days after you started your period). Based on this information, your app may tell you that your fertile window for the current cycle begins on cycle day 10. But what if your fertile window actually ends up starting on day eight? If you’re trying to avoid pregnancy, you’d be putting yourself at risk by trusting the prediction. And if you’re trying to get pregnant, you may end up missing an opportunity to conceive. If you chart your fertility, then you’ll be able to recognize the error and adjust accordingly. But what about women who fully trust fertility predictions simply because they haven’t been taught how fertility really works? Eek.


Sign up for our FREE 7-day mini-course to learn all the stuff sex ed SHOULD have taught you about your period, how to spot a problem, and how tracking your periods can improve your health!


Early ovulation

Another potential failure point when trying to predict fertility is early ovulation. Let’s say you don’t typically start producing cervical fluid – and, thus, aren’t fertile – until day 13. However, this cycle you unexpectedly ovulated on day 12. According to a fertility predictor, you’re not even supposed to be fertile yet! This is a HUGE problem if you’re relying on it to avoid pregnancy. On the flip side, if you’re trying to conceive, you might miss your fertile window entirely.


If you have PCOS, fertility predictors are particularly dangerous. Since predictions are typically calculated based on your average cycle length, there’s a pretty huge opportunity for errors if you have highly irregular cycles. If you struggle with PCOS, my advice is to ditch the period and fertility predictors altogether. Charting your fertility using the sympto-thermal method of fertility awareness does amazing things to help shed light on what’s happening with your cycles.

Shorter-than-typical luteal phase

The last failure point that I’m going to discuss involves women with relatively short luteal phases. Say, for example, that your cycles are typically 30 days long and your luteal phase (or post‐ovulatory phase) is 11 days long. Many predictors will calculate (read: predict) the end of your fertile window based on your average cycle length and population averages for luteal phase length. Since most women have a luteal phase that falls between 12 and 16 days, you’re basically SOL if you have an 11‐day luteal phase. If you do have a shorter‐than‐typical luteal phase, an app that predicts fertility might tell you that you’re no longer fertile even though you actually are. That’s a pretty great recipe for trouble, right there.

One of the biggest problems with apps that claim to predict fertility (besides the fact that they can’t see into the future) is that you won’t know what information is being used to make the predictions. This means that the margin of error could be enormous. Even if an app spells out exactly how your predictions are calculated, predictions should never be relied on. The only way to really know that you are or aren’t fertile is to practice the sympto-thermal method and to chart your fertility signs using an app like Groove or some other means…no guesswork necessary.

Women’s cycles are all unique, and they shouldn’t be placed into one big generalized box. That’s just not the way the human body works. So until we get our hands on shiny crystal balls that peer into our menstrual future, steer clear of fertility predictors.

Jennifer Aldoretta is the cofounder and CEO of Groove. She is an entrepreneur, engineer, and biohacker who is obsessed with periods, nutrition, hormones, and the microbiome.