Most manufacturers claim that their tests are 97 to 99 percent accurate, but a test can be less effective if it's not used properly, used beyond its expiration date, or exposed to sunlight. To be sure you get the best results, read the instructions thoroughly before starting the test and follow them exactly.
Home pregnancy tests work by detecting the presence of the hormone human chorionic gonadotropin (hCG), which shows up in small amounts in your urine shortly after you get pregnant. (That's why the tests have you pee on a stick or into a small container.)
It's best to test first thing in the morning, but if you're testing later in the day, wait a few hours between bathroom breaks to be sure your bladder is full. Contrary to popular belief, you shouldn't drink large amounts of liquid before the test. This actually dilutes the hCG in your urine, which can create what's known as a false negative (where the test says you're not pregnant—usually because hCG levels are too low to be detected yet—but you really are).
The instructions should also indicate whether certain medications—such as many prescription fertility drugs, for example—may trip up test results. Taking these can lead to a false positive result (where the tests say you're pregnant, but you're really not). If you're using these medications, your doctor will likely want to test in the office to ensure an accurate reading.
If you get a positive result, call your doctor right away. He or she will want to schedule an appointment to make sure you and your baby are healthy and developing on track. FYI: False positives are very rare for women not taking fertility drugs, so there's usually no need to test again once you spot a positive result.
If you get a negative result but still have not gotten your period several days later, you should test again. Home pregnancy tests aren't always accurate; levels of hCG can be very low early on in pregnancy, and the test you used may not be sensitive enough to detect hCG until a bit later.
If after that Aunt Flo still hasn't arrived and test results still return negative, further testing may be necessary. "Irregular menses can and should be evaluated with blood tests to determine the cause," says Marra Francis, M.D., a board-certified obstetrician and gynecologist. "A serum pregnancy test should be also run at that time to determine if pregnancy is the reason despite negative urine tests."