Hey shoneen, how are you doing?
It is advisable to wait at least a week after a missed period for a reliable answer to that question, but often the suspense is often just too much to bear! This may be related to delayed ovulation in that particular cycle, or because the pregnancy test was not sensitive enough.
An early morning urine is usually most concentrated will improve the pickup. The earlier that pregnancy is detected, the more miscarriages are found.
Best wishes,