According to the American Pregnancy Association, most pregnancy tests are effective by the first day of a missed period. First Response says that some tests are more sensitive and can show a positive result up to six days before a missed period.
The American Pregnancy Association says that early pregnancy tests are more likely to show a false negative, which occurs when the body's hormone levels are too low to register pregnancy on the test. In this case, the user should wait a few days and take the test again. False positives are rare, and a positive result usually indicates pregnancy.