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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?
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Chances of a false positive pregnancy test
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.”
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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.”
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.
What is a pregnancy test? | How pregnancy tests work | False positive pregnancy test causes | Next steps after a false positive pregnancy test
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.
"[This happens to] women who are very motivated and trying to conceive, and they will often do the pregnancy test early and see a positive result, which is obviously very exciting for them initially," Zev Williams, M.D., associate professor of Obstetrics and Gynecology and chief of the Division of Reproductive Endocrinology and Infertility at Columbia University Irving Medical Center, tells SELF. "But it just might be the result of the fertility medication that they took. It's something we caution patients about so they don’t get false hope when they see that test is positive."
4. You still have hCG in your system after a delivery or miscarriage.
Most miscarriages occur in the first 13 weeks of pregnancy5. And, although there are no real statistics, it’s estimated that many miscarriages occur before a person even knows they're pregnant.
After you give birth or miscarry, hCG can persist in your body for months, Dr. Williams says. One study published in The Journal of Clinical Endocrinology and Metabolism estimates that it takes four to six weeks for hCG levels to return to normal after a person has a miscarriage6. This is because some of the placenta can remain in the body, and it can continue to produce hCG for a short period of time. "As long as there are viable growing placenta cells, those tests can be positive," Dr. Williams says.
5. You had an ectopic pregnancy.
An ectopic pregnancy happens when a fertilized egg ends up growing outside the uterus—usually in a fallopian tube—instead of inside the uterus7. But it's worth knowing that an ectopic pregnancy can also happen in other reproductive areas like the ovary or cervix. An ectopic pregnancy isn’t viable since the embryo isn’t inside the uterus where it can grow and thrive. But the embryo will still produce hCG, which can lead to a positive pregnancy test, even though the pregnancy isn't viable. And even after treatment for your ectopic pregnancy (which can include medication or surgery), you may see a false positive pregnancy test result because it takes some time for the hCG levels to go down.
An ectopic pregnancy can be life-threatening8. Call your doctor right away if you experience any of the following symptoms:
- Severe pain in your abdomen or pelvis while pregnant
- Vaginal bleeding (that’s out of the ordinary)
- Severe dizziness, lightheadedness, or fainting
- Shoulder pain
- An urge to poop
- Any other symptoms that are making you feel worried
6. You've had an extremely early miscarriage.
We're putting this reason last because it's not technically a cause behind a false positive pregnancy test, but it's still worth covering because many people confuse it with one. You might get your period a few days after a positive pregnancy test and assume you had a false positive result. However, it could be that you were pregnant but miscarried very early on. “It’s not that the test is wrong—it was completely accurate at picking up the hormone [hCG]," Dr. Williams says. "Unfortunately, this wasn’t a healthy pregnancy." Doctors refer to this early pregnancy loss as a chemical pregnancy.
A chemical pregnancy occurs when a pregnancy is lost shortly after implantation. People who have a chemical pregnancy often experience bleeding (when they miscarry) right around the time of their expected period, which is why it’s easy to assume you just had a false positive test result.
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Here are some next steps to consider after a pregnancy test result.
While most pregnancy test results are accurate, it can be hard to know right away if you’re dealing with a false positive pregnancy test result. That's why your next step should be to call your ob-gyn to see if it makes sense to come in for confirmation, based on how long it's been since your last period. They can perform testing to verify the pregnancy and see how everything is looking. Dr. Abdur-Rahman says he views at-home pregnancy tests more as "screening tests"—a blood test is a diagnostic test that can actually confirm you're pregnant. If a blood test confirms you're pregnant but you're experiencing any symptoms such as bleeding or pain, your ob-gyn can continue to monitor hCG levels in your blood to make sure the pregnancy is developing properly before an ultrasound can reveal more details.
And if you get a negative pregnancy test, know that a false negative is sometimes possible too. If you take an at-home pregnancy test too early, it might not pick up the hCG hormone. But, as we explained above, those hormones should double every 72 hours with a healthy pregnancy. "The longer you wait, the more likely you are to detect the pregnancy,” Dr. Williams says. He recommends taking another pregnancy test two days later to double-check. (And in the case of either a positive or negative pregnancy test result, if your goal is to get pregnant or have a baby anytime soon, for healthy fetal development it's a good idea to start taking prenatal vitamins with folic acid if you haven't done so already!)
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Sources:
1. Mayo Clinic, Getting Pregnant
2. Cleveland Clinic, Your Guide to Pregnancy Tests
3. Mayo Clinic, Getting Pregnant
4. First Response, Early Pregnancy Test Result
5. American College of Obstetricians and Gynecologists, Early Pregnancy Loss
6. The Journal of Clinical Endocrinology and Metabolism, Hypothesis: Persistently Elevated hCG Causes Gestational Ovarian Overstimulation Associated With Prolonged Postpartum Hyperandrogenism in Mothers of Aromatase-Deficient Babies