Avoiding the Dreaded Period Sneak Attack: the Secret to Accurate Period Predictions

wrote this on December 12, 2014

Your period is still a week away—or so you thought—and suddenly…period sneak attack! Sound familiar? This is an experience I think a lot of us can relate to, and it has happened to me more times than I’m comfortable admitting (roughly a zillion).

The way many of us (and our apps) predict our next period is extremely unscientific. I’ve blindly circled days on a calendar more times than I can count, and I’ve tried tons of apps that use all-too-basic math to (unsuccessfully) predict my periods. The lack of science around our periods means a huge lack of preparedness for aunt flow, which can result in some pretty unsightly stains. There’s nothing worse than frantically scrambling to find the nearest pad or tampon—except, perhaps, shoving a giant wad of toilet paper in your underwear until your scrambling pays off.

I’m here to tell you that there IS a better way to predict your periods, and it might just change the way you think about your body (it did for me). What I’m about to reveal would put your sex ed class to shame. So what’s the big secret? It’s actually something you’ve been doing for most of your life every time you get sick: taking your temperature! The best part? It’s actually ACCURATE. Maybe there’s a period god, after all.

How does temperature help with period predictions?

I’m glad you asked! Your body temperature can actually tell you a lot more than whether or not you’re sick; it can also let you in on what your hormones are doing.

By measuring your body temperature each morning when you wake up—which is called your basal body temperature—you can detect your day of ovulation. Since the day of ovulation can be affected by different stresses (physical, mental, emotional), trying to predict ovulation—or relying on an app to predict it—simply doesn’t work. However, measuring your basal body temperature allows you to retroactively detect when you ovulate. And once you know you’ve ovulated, predicting your next period is ridiculously easy.

Before you ovulate, your basal body temperature is low. After you ovulate, a hormone called progesterone makes your temperature rise and stay high until you get your period.

The number of days between ovulation and the first day of your period (which is represented on the chart by the smaller numbers circled in black) can be anywhere between 11 and 16 days, but it will be almost exactly the same length every single cycle. For me, there are about 12 days between ovulation and my next period, so once I see that my temperature has gone up, I know exactly when my period will start. Your temperature will also usually drop back down on the day you start your period, which is extra helpful.

This makes period prep a cinch and helps you avoid the dreaded period sneak attack. The amazing thing is that this method works even if you have irregular cycles or suffer from PCOS.

If you’re interested in taking advantage of this awesome little nugget of period info, you can read up on which type of thermometer is the best for measuring basal body temperature or even purchase a digital basal thermometer from our online store.

If you’re ready to take the plunge and get started with super accurate period predictions, download the Groove app.

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.