My periods were due on the 11th of September. Today is 17th and I have still not got my periods.I have been checking with urine pregnancy kit since 12th daily till today but the results were negative. Is it too early to trust the result or is it too early to confirm.If I am not pregnant why am I not gwtting my periods.. please help.

The accuracy during the first week is under 50%. It increases to over 90% at the end of the second week.