Pregnancy test kits are designed to detect levels of hCG produced by your body if you are pregnant. Your body may be producing hCG before your period is due, however it is ideal to test on the first day of your missed period. If you were to test earlier than the first day of your missed period, the result may be incorrect. It may show a negative result if your hCG levels are not yet high enough to detect the pregnancy, or it may show a positive result and not develop into a true pregnancy due to the incidence of spontaneous abortions, which can occur before your period is due.