An image of a woman holding a pregnancy test.

Home pregnancy tests work by detecting the presence of the hormone human chorionic gonadotropin (hCG), which shows up in your urine shortly after conception. They're available at grocery stores and pharmacies, and they're relatively easy to use. But how accurate are home pregnancy tests, and what external factors can affect the results? Here's everything you need to know. 

How Accurate are Pregnancy Tests?

Home pregnancy tests claim to be about 99 percent accurate. Indeed, with proper use, they can detect results just as well as the urine tests you'd take at a doctor's office. Appropriate timing is crucial for getting a reliable reading.

The best time to take a pregnancy test is after missing your period. Here's why: About six to 10 days after fertilization, the embryo implants in the uterine wall, and your body starts producing hCG. It will take some time to make enough hCG for a positive pregnancy test, says Daniel Roshan, M.D., an OB-GYN at Rosh Maternal-Fetal Medicine in New York City. The farther along you are in pregnancy, the greater your hCG levels will be, so waiting until you miss your period increases the chances of accuracy. (If you don't have regular periods, the rule of thumb is waiting three weeks after having sex.)

Regardless of the brand used or result obtained, most manufacturers recommend repeating the process a few days later to confirm results, because levels of hCG following conception are so low. The strength of each test varies, and the test may not pick up the amount of hCG hormone present the first time you test.

It's important to note that a pregnancy 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.

What About Early Pregnancy Tests?

Some "early detection" pregnancy tests claim to work before you miss your period. However, the results likely won't be as accurate. Every woman has varying levels of hCG during pregnancy; the early tests may detect pregnancy for some women and not for others. Always read the packaging of your pregnancy test for more information about timing and accuracy. 

Can You Get a False Negative or False Positive Test?

Your hCG levels are very low early in pregnancy and ramp up as your pregnancy progresses. Because of this, it's possible to get a negative test result (but actually be pregnant) if you test too early. This can also happen if you don't follow the instructions properly.

If you suspect that you've done the test too early, before there's enough hCG in your urine, wait a few days and repeat the test. If it's still negative and you haven't gotten your period, something else may be happening in your body. "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. Certain medications and illnesses can cause you to skip a period—and so can excessive exercise, low body weight, early menopause, birth control pills, or stress.

False positives are very rare for women not taking fertility drugs. Nonetheless, they can happen if your pregnancy ended shortly after conception (chemical pregnancy), you have ovarian tumors, you had a recent miscarriage or abortion, or your pregnancy test was expired.

How to Increase Accuracy of Home Pregnancy Tests

To ensure your pregnancy test is as accurate as possible, consider the following tips and guidelines.

Test in the morning. It's best to test first thing in the morning because hCG is more concentrated in your urine when you first wake up. If you're testing later in the day, wait a few hours between bathroom breaks to ensure your bladder is full. 

Don't chug water before testing. 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.

Check if your medications can skew results. The pregnancy test instructions should indicate whether certain medications—such as prescription fertility drugs—may trip up test results. Taking these can lead to a false positive result. If you're using these medications, your doctor will likely want to test in the office to ensure an accurate reading.

Use the product correctly. Inaccurate results may be due to improper usage, an expired product, or exposure of the test to sunlight.

The Bottom Line

Home pregnancy tests are around 99 percent accurate when used correctly. If you get a positive result, call your doctor right away. They will want to schedule an appointment to make sure you and your baby are healthy and developing on track. 


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