After How Many Days Of Missing Periods Does UPT Show Accurate Result?

Dear doc,my periods due date was on 30th December,my cycle length is 31 days,i didn't get my periods yet,i have done a upt with prega news on 31st December but result was negative,how do i know if am pregnant or not??i couldn't even sleep thinking these..when can be a upt shows correct result after the due date??