The fertility prediction feature works by using averages of your past data, including your temperature shifts and average cycle lengths.
An incorrect temperature shift marked in your chart can throw off your fertility prediction. Remember, a temperature shift is defined as a temperature which is at least .2 degrees higher than any temperature in the previous 6 days.
If your fertility prediction seems off, the first step is to check your temperature shifts on your charts. Does one look off? If you go to your Data screen and remove the temperature shift that was marked incorrectly, this will make a big difference! (Just tap on Temperature Shift again to remove it).
If you'd like more information about how to identify your temperature shift, take a look at these links :)