When you think you might be pregnant, you really want to know—like, now. Maybe you have some of the symptoms of pregnancy, like sore breasts, or perhaps you’ve missed your period completely. So you decide to pick up an at-home pregnancy test at the store.
But even after you pee on the stick, you might still be wondering: Is this thing even accurate? What if it’s a false negative—or positive? How likely is that?
Since the first at-home pregnancy test hit the market in 1976, people have been questioning their reliability. Here's what you need to know.
How Home Pregnancy Tests Work
At-home pregnancy tests attempt to detect the presence of human chorionic gonadotropin (hCG) in your urine, according to the National Women's Health Information Center. This "pregnancy hormone" is made when a fertilized egg implants in the uterus. For most women, implantation occurs about six days after the egg is fertilized by the sperm, but for others, it can occur as late as the first day of the missed period. Each day after implantation, the amount of hCG in your system will increase rapidly.
So really, the most accurate pregnancy test is the one you wait to take after you miss your period—giving time for hCG to build up. But what about tests that claim they can detect pregnancy days before your missed period? The harsh truth is that there are currently no standards used to evaluate the claims on pregnancy test packages, and these early tests are often not as accurate as they say.
But in general? If used correctly, most home-pregnancy tests are pretty reliable, according to the Mayo Clinic.
Errors That Can Occur With Home Pregnancy Tests
There are two ways a home pregnancy test can be inaccurate:
- A false positive. This means that although your pregnancy test indicates that you are pregnant, you actually are not.
- A false negative. This means that although your test says that you are not pregnant, you really are pregnant.
False positives are more rare than false negatives: A positive test is almost always a pregnancy.
Possible Reasons for a False Negative
Here are some potential reasons why this could happen, per the Mayo Clinic:
- You tested too early. It takes about five to six days after fertilization for an embryo to implant in the uterus, according to UCSF Health—and in some women implantation doesn’t occur until a day (or several) after the missed period. If you take the test too early, your hCG levels might not be detected due to implantation occurring later than usual.
- The urine is too diluted. It’s recommended that you test for pregnancy first thing in the morning, the first time you have to pee. That way your urine is the most concentrated and it will be easier to detect hCG.
- You’ve miscalculated your period. If you made an error in calculating when your period was actually due, it can lead you to take a test too early, when hCG levels aren't high enough to detect.
- You check the results too soon. Follow the directions in your kit and wait the allotted amount of time before reading your results.
Possible Reasons for a False Positive
It’s less likely that you will receive a positive result when you really are not pregnant—but again, it can happen. Here’s why it might, per the Mayo Clinic:
- Your meds are interfering. Some medications may cause you to have a false-positive home pregnancy test result. These drugs include the fertility treatment drugs that contain hCG themselves, or human menopausal gonadotropin (hMG) drugs such as Metrodin, Pergonal, Repronex
- You have a certain medical condition. Some conditions such as rare cancers, trophoblastic disease, ovarian cysts, and thyroid disorders can sometimes create false-positive results.
- The egg doesn’t implant properly. It’s estimated that up to one-third of all conceptions implant and then fail, perhaps because the embryo doesn’t implant properly or spontaneously aborts soon after implantation (this is a very early form of miscarriage). You may produce some hCG even though a clinical pregnancy didn't develop. Doctors sometimes call this scenario a “chemical pregnancy.”
- You let the test sit too long before reading the result. If you read the test results after the instructed time period, a faint positive may appear, which can be inaccurate if read at that time.
The Bottom Line
If you’re taking a home pregnancy test, make sure to follow the directions precisely. Always read the results in the time frame indicated or they may be inaccurate. (Use your phone timer!) And check the expiration date on the box to make sure the test you're using is still effective.
When in doubt about your results, wait a week and test again (it’s always wise to buy two tests). Or go into see your doctor to receive a blood test or ultrasound that checks for pregnancy—these are the most accurate way to confirm your results, according to the Mayo Clinic.