Maybe you can’t wait to have a baby. Or maybe that’s the last thing you’re hoping for.
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Either way, if you think you might be pregnant, you need to know for sure. But what are the odds a positive pregnancy test might be wrong?
“Home urine pregnancy tests are pretty reliable,” says Ob/Gyn Jonathan Emery, MD. “But there are some reasons you might get a false-positive result.”
Dr. Emery explains when and why a pregnancy test might give a false positive — and what you can do to make sure the stick doesn’t lie.
How do pregnancy tests work?
When you get pregnant, your body produces a hormone called human chorionic gonadotropin, or hCG. Home pregnancy tests look for that hormone in your urine. If hCG is present, you should get a positive test result when you pee on a stick.
But hCG levels start out very low and increase over time. If you take the test too soon after conceiving, it might say you’re not pregnant when you really are.
In other words, timing can lead to a false negative. But what about a false positive?
False positives aren’t super common, Dr. Emery says. But they’re not impossible. Some potential causes include:
Early miscarriage or chemical pregnancy
You took a pregnancy test and got two lines. (Positive!) But a few days later, your period arrived in force. What gives? The most common reason this happens is an early pregnancy loss, also known as a chemical pregnancy. In this case, the test was accurate — there was a pregnancy, but it wasn’t a viable one, Dr. Emery explains.
“It’s not technically false since a very early pregnancy did occur,” he says. “But this is the most common reason that a pregnancy test might appear to have been false.”
Fertility medications
“A lot of fertility treatments involve taking hCG injections,” Dr. Emery says. If you’ve been taking fertility medications, that hCG might still be floating around your system.
That could trigger a positive pregnancy test, even if you’re not pregnant. To avoid that mix-up, wait at least two weeks after your last hCG injection to take a home pregnancy test, he says.
Recent pregnancy
If you were recently pregnant, you might still have leftover hCG in your system. After childbirth, miscarriage or treatment for ectopic pregnancy, the hormone can remain in your body for up to four to six weeks, Dr. Emery says. “That could lead to a positive pregnancy test when your body just hasn’t cleared the hCG yet.”
User error
Home pregnancy tests aren’t especially hard to use. But you still have to pay attention since a mistake can lead to incorrect results.
If you wait too long to read the results, for example, or use more drops of urine than the test calls for? You might want to take the answer with a grain of salt. “If you don’t follow all the instructions, any results — positive or negative — could be false,” Dr. Emery says.
Pregnancy test accuracy: How to get results you can trust
Luckily, false positives are rare. And there are steps you can take to make sure your home pregnancy test gives you results you can trust.
- Time it right. Don’t take a test too early. It’s more likely that you’ll get a false negative — or that the test will detect a chemical pregnancy that isn’t able to progress. Dr. Emery recommends waiting until the day of your missed period, or a few days later, to take the test. “Timing is important. The test is most accurate if you wait at least until the day of your expected period,” he says.
- Follow the directions. Make sure your test isn’t expired. Read the directions before you start. And follow the steps exactly to avoid a stressful false result.
- Repeat it. If you got a positive home pregnancy test, you might be eager to get a blood test to confirm the result. Blood tests are more accurate, so that’s certainly an option. But it isn’t always necessary, Dr. Emery says. “If you get a positive result from a urine test, then repeat the test in three to five days. If it’s still positive, you can trust the result,” he says.
Home pregnancy tests are inexpensive, private and quite reliable, Dr. Emery adds. “People often don’t believe what they’re seeing. But if you’ve used the test correctly and done it at the right time, it’s probably true.”
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.