What can cause multiple false positive pregnancy tests

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.

Yes! Here's everything you need to know about why that second line might appear when you're not actually pregnant.

I’m no stranger to premenstrual symptoms, but a few months ago, I experienced a couple that were new to me: My breasts were tender and my lower back throbbed with pain. I put it out of my mind until my cycle-tracking app alerted me that my period was a few days late—at which point I threw my three kids in our minivan and raced to the store for a pregnancy test. Back home, as my toddler tapped incessantly on the bathroom door, I watched as a faint second line appeared. This can’t be happening, I thought. My husband had undergone a vasectomy three years earlier.

Over the next few days, the shock settled and turned into excitement. Then, nearly a week later, I started cramping and bleeding. My doctor asked me to head to the hospital. Hours after waiting in the ER, a mandatory mask covering my fear and worry, the physician finally entered my room. “I’m sorry to tell you that you aren’t pregnant,” she said. My blood test, she explained, showed not even a hint of the hormone hCG—meaning I wasn’t just not pregnant, but I had never been pregnant at all. It seemed the test I’d taken at home had produced a false positive, and my symptoms—the PMS, the bad cramping, the lateness—despite being unusual for me, were simply part of my monthly period.

How at-home pregnancy tests work

At-home pregnancy tests work by indicating whether hCG (human chorionic gonadotropin) is present in your urine, says Mary Coll-Black, an OB/GYN at St. Joseph’s Healthcare in Hamilton, Ont. The presence of hCG confirms a pregnancy, and a lack of it suggests the individual is not pregnant. If, like me, you have a positive test but you are not, in fact, pregnant, then you either have hCG in your body for another reason or something went wrong with the test.

Why the test found hCG

It’s quite rare for hCG to be detectable in your urine or blood if you aren’t pregnant, but there’s a laundry list of reasons why it could happen: if you’ve recently given birth, had an abortion or experienced a pregnancy loss, or had fertility treatments where hCG was injected. It typically takes weeks for hCG to fully leave your system, says Yolanda Kirkham, an OB/GYN at Unity Health and Women’s College Hospital in Toronto. There are also some medications that could potentially create a false positive, she says, like aspirin, the anti-seizure medication carbamazepine, and methadone. There’s even some medical literature that suggests women could end up with a false positive if they test immediately after having sex, says Kirkham, because there may be a trace amount of hCG in semen.

Testing problems

A false positive can occur if you haven’t properly followed the instructions on the pregnancy test. For example, you’re meant to check the test results within a few minutes, while the urine is still wet, because as it dries, an evaporation line may appear as a faint second line. (You may see this referenced in online mom chats as an “evap line.”) An expired test also has the potential to skew the results; a cheap test, on the other hand, does not. Kirkham confirms that pricey tests aren’t more accurate than inexpensive ones. Even a cheap, dollar-store pregnancy test will not increase your chances of a false positive.

Figuring it out

To get to the bottom of a false-positive pregnancy test, your doctor may ask questions and review your history. In my case, there was no reason why hCG should have been be present, a follow-up blood test showed no hCG, and I’m confident I used my pregnancy test correctly. My husband underwent a semen analysis to ensure his vasectomy had been effective (it was). The most likely scenario, said both the ER doc and my family doc, is that I had a true, unexplainable false positive—which, by the way, almost never happens. “A positive pregnancy test that turns out to be false is extremely rare,” says Coll-Black. “It’s not something we come across often.”

I’ll never know what that second line on my test was all about. What I do know is that it really mixed me up emotionally. Fortunately, most women will never go through what I did, since at-home pregnancy tests, says Kirkham, are 99 percent accurate.

The flip side: False negatives

If your pregnancy is in its early stages and your body has not yet produced enough hCG for the pregnancy test to detect, you could get a false negative, which means you’re actually pregnant even though the test says you’re not. The earlier you test, the higher the chances that this scenario will occur. If you get a false negative but still suspect you’re pregnant, repeat the test in a few days. If it’s negative again but your period still hasn’t arrived, book an appointment with your healthcare provider.

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