Pregnancy test timing can be complicated. Let's start at the beginning so you understand how they work.
Pregnancy tests are designed to detect the hormone 'human chorionic gonadotropin' (hCG). This is known as the 'pregnancy hormone' and is generated by your body when you are pregnant. hCG is not generated in the body until implantation takes place. Since the timing of implantation - and even ovulation - varies not only from woman to woman, but month to month, taking even a sensitive test too early can give misleading results.
There are some tests sensitive enough to detect hCG levels prior to a missed pregnancy - hCG levels double every couple days in early pregnancy - but if you are looking for accurate results you're better to try to wait until you have missed your period before you take a pregnancy test. If you take the test too early, you may indeed be pregnant (or about to become pregnant), but the test will tell you tha you aren't (a 'false negative').
Additionally, many early pregnancies end in miscarriage. Often you are not even aware that you were pregnant if this happens. Then you miscarry around the time your period was due. You simply think you were late and that your period is a bit heavy. Thus if you take the test too early, you run the risk that you will be burdened with the knowledge of a miscarriage you otherwise would not have known about.
All in all, it is better to just wait until you have missed your period to run a pregnancy test.