The best time to do the Predictor test depends on your menstrual cycle. Regular cycle - Your cycle is regular if there are always the same number of days between the beginning of your period and the start of the next one. You can therefore use Predictor on the first day you are theoretically late. Irregular cycle - Your cycle is irregular if the number of days between the beginning of your period varies. Sometimes your period arrives 27 days or 29 days later. In this case, base yourself of the longest cycle in the last six months. Just to be sure, add an extra day.