tests measures the amount of human chorionic gonadotropin(HCG
) in the body to detect the pregnancy.
These are of two types, blood tests and urinary tests.
What you have done is urinary test it will detect HCG levels in the urine. But HCG should reach certain level to be detected by urine test.
If more sensitive test kit is used you can detect the pregnancy at the time of your due date but if less sensitive test kit is used better to perform the test 1 week after missed period.
Now what you can do is repeat the test or go for blood test which will give more accurate result with less levels and even before the due date.
For blood test you have to consult the doctor.