here is what I've found,
Many home pregnancy tests (HPTs) claim to be 99 percent accurate on the first day of your missed period. But research suggests that most HPTs do not always spot pregnancy that early. And when they do, the results are often so faint they are misunderstood. If you can wait one week after your missed period, most HPTs will give you an accurate answer. Ask your doctor for a more sensitive test if you need to know earlier.