What could cause a false positive pregnancy test

Learn the reasons for a false-positive test result

Is the stick always right? Usually yes, but sometimes no. Learn about the rare reasons for a false-positive pregnancy test.

A home pregnancy test has confirmed what you suspected — you’re pregnant. Home pregnancy tests work by detecting the presence of the pregnancy hormone hCG in your urine. Their results are considered reliable, especially when you wait for at least a week after you miss your period to use one.

They’re not foolproof, however. It is possible to get a positive result when you’re not actually pregnant, which is called a false-positive. If you’ve been looking forward to a positive pregnancy test result, a false-positive is disappointing. The good news is they’re rare. The best thing to do when you’ve gotten a positive result is to contact your doctor. A blood test will confirm that you’re pregnant.

4 Causes of a False Positive Pregnancy Test

The four main causes that may lead to a false positive pregnancy test include a chemical pregnancy, a recent miscarriage, medication or a defective test. Read below to learn more about each of these scenarios.

Chemical pregnancy

A chemical pregnancy is a pregnancy, but an unhealthy one that results in an early miscarriage. A chemical pregnancy occurs when an egg has fertilized but doesn’t attach to the uterus. Usually, you will have a heavy period soon after your period is normally due. It’s estimated that 25-40 percent of pregnancies end as undetected miscarriages.

Recent miscarriage or birth

If you’ve had a baby recently or a miscarriage, you may test positive for pregnancy. When you’re pregnant, hCG levels rise quickly and peak at about 10 weeks. When a pregnancy ends, the levels go down, but slowly. The hormone can remain in your blood and urine for up to six weeks after a pregnancy ends.

Medications

If you’re undergoing fertility treatments to become pregnant, you may be getting hCG injections to stimulate ovulation. The medication can cause a false-positive result if you take the test too soon after a missed period.

Medications that may cause a false positive pregnancy:

• Antihistamines
• Antianxiety medications
• Antipsychotics
• Diuretics
• Parkinson’s disease medications
• Methadone

Defective pregnancy test or user error

Before buying a home pregnancy test, check to make sure it hasn’t expired. If it has, the chemical used to detect hCG may not work. It’s important to read and follow the test instructions carefully. For instance, some tests require leaving the dipstick in the urine stream for a specific amount of time. Taking the test too early can also result in a false-positive.

How to Avoid a False-Positive Pregnancy Test

The best way to avoid a false-positive pregnancy test is to wait at least a week after you’ve missed your period to take the test. The level of hCG hormone will be higher and easier to detect. Detecting a chemical pregnancy before it miscarries can be emotionally hard.

Many home pregnancy test kits include two tests. If you get a positive result, you can wait a couple of days and use the second test. Then contact your doctor for a blood test to confirm the good news!

Looking for an OB/GYN? We can help!

No matter what, taking a pregnancy test can be an emotional roller coaster. When it comes to getting a false positive pregnancy test result, the experience can be deeply upsetting, particularly for anyone who has had a difficult time getting pregnant or is actively hoping to get pregnant (or both). The hope that comes with seeing that positive sign crashing down as you realize it was a false positive pregnancy test? It's a uniquely devastating emotion. 

Of course, getting a false positive pregnancy test when you’re actively trying not to get pregnant can be just as horrible. Yup, you could probably do without the panicked verification trip to the ob-gyn, just to learn there was nothing to stress over. 

Thankfully, experts note that false positive pregnancy tests are rare. Unfortunately, though, they can happen. Here are some of the most common causes of a false positive pregnancy test, along with some additional information that can help you put it all into context.

First, what is a pregnancy test?

Let's start with the absolute basics just to make sure we're all on the same page. A pregnancy test tells you if you’re pregnant or not. That's simple enough to understand. But it's understandable if you're a little fuzzy on the details of what these devices actually look for to identify a pregnancy.

At-home pregnancy tests check for the hormone hCG. HCG is short for human chorionic gonadotropin, which the body creates during pregnancy. Right after a fertilized egg attaches to the uterine lining, the placenta then forms and starts producing hCG1. A blood test can detect hCG about nine days after conception, and a urine test can detect it 12 to 14 days after conception, according to the Cleveland Clinic, although it varies—some especially sensitive urine tests can detect a pregnancy even earlier. A person’s hCG level typically doubles every 72 hours through 8 to 11 weeks of pregnancy. Then it remains consistent and starts to go down after delivery.

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How does a pregnancy test work?

An at-home pregnancy test is designed to find hCG in pee2. If the hormone is present, it triggers a chemical reaction and the test signals that you're pregnant. If hCG is not present, the test will say you aren't pregnant. Many tests use two lines to mean you’re pregnant and one line to mean you’re not—but it depends on the test brand. Some tests use plus and minus signs. Some digital pregnancy tests have a screen that plainly reads "pregnant" or "not pregnant." Which can save you some “Is that one line or two?!” squinting, at least.

Seems simple, right? As Ina Garten would say, "How easy is that?" Typically, pretty easy. Most at-home pregnancy tests claim to be about 99% accurate3. Ultrasounds typically can't detect a pregnancy until a little later in your pregnancy and that's why at-home tests are so useful. But, sometimes, other elements can mess with a pregnancy test's results and tell you you’re pregnant when you’re really not. 

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What can cause a false positive pregnancy test?

Here are the most likely reasons you may end up with a false positive pregnancy test:

1. You let the test sit too long before looking.

If you take a standard pregnancy test with line indicators, it's important to check the results according to the specific instruction of the test. If you let it sit too long before reading the results, urine on the test can evaporate and make it look like you have two lines instead of just one. "Oftentimes people will see evaporation lines as urine starts to evaporate off the test," Jamil Abdur-Rahman, M.D., board-certified ob-gyn, tells SELF. That might cause the test to look like it has a faint second line—making it positive—but it really only has one.

The best way to avoid this: Read the pregnancy test's directions and follow them exactly. The popular pregnancy test brand First Response, for example, instructs users to wait three minutes after taking the test, then read it as soon as possible4.

2. The pregnancy test is expired.

The second most common reason Dr. Abdur-Rahman's patients get false positives is because the test is expired, he says. When a test is past its expiration date, the chemical that detects hCG doesn't always work as it should, and you’re more likely to get a misread. "The test can expire and the chance of having a false positive increases," Dr. Abdur-Rahman says.

3. You’re on fertility medications that raise hCG levels.

If you take a pregnancy test too soon after taking a fertility drug that contains hCG—like some injections that are often part of in vitro fertilization—you could get a false positive.