To get the most accurate result, it would be best to wait at least five days after your missed period. There are some test that say you can take the test up to five days BEFORE your period, but it's only about 60% accurate at that point. I know it's nerve wracking, but the best thing to do is wait.
When you don't have your period most of the time that's a dead giveaway. If you had unprotected sex, if the condom broke. Or if your on Ask posting questions like can I still get pregnant if I hang up -side-down? Get a pregnancy test, and see a Dr..