There are many great reasons to use a period tracker these days. Maybe you want to know when you’re ovulating. Or you can never remember for the life of you when your period is coming. Whatever the case may be, we’ve got bad news. Period trackers aren’t very reliable, as new research shows that they’re not as accurate as we might think they are.

The University of Washington conducted research on 687 people and set it alongside data gathered from 2,000 reviews from the most popular period tracking apps, such as Clue, Glow, Period Pad, and Eve. The findings weren’t exactly positive.

Researchers found that, although the standard period tracker allows women to track the days of their period and their symptoms, it doesn’t let you enter in any extenuating circumstances that might affect the arrival of your next period. Which means the prediction for your next period and ovulation will probably be a little off.

Study author and independent researcher Nikki Lee told the Independent that the algorithms of period tracking apps aren’t strong enough to cater to irregularities.

These apps certainly shouldn’t be used in place of birth control, because there’s no guarantee that it will correctly determine the days that you’re ovulating. If you have any questions about your period or you’re wondering why it’s so irregular, your best bet is to speak with your doctor, rather than relying on an app that is proven to be anything but reliable.