The monitor provides an estimate of your period due date using information it has available, e.g. when your Peak days were or previous cycle details. If this information is not available it works it out based on a 28-day cycle. This is only estimation. If you know your cycle is not normally 28 days you can test for pregnancy when you think your period is due. If you test when your period is due and get a ‘Not Pregnant’ result, test again in 3 days. If this is still ‘Not Pregnant’ and your period has not started you should consult your doctor.