What could cause a positive pregnancy test

A pregnancy test can show a false positive. Although this is a rare occurrence, this most frequently happens with home pregnancy tests due to errors when using it or due to expiry.

Another common cause for a false positive is a chemical pregnancy. This occurs when the egg has been fertilized, but is unable to properly implant on the uterine lining. It is unable to develop into a full fetus, but the body initiates the production of hormones anyway, giving the test a positive result. A pregnancy that does not continue however will eventually show a negative pregnancy result. 

In addition, many women who undergo fertility treatments with hCG injections will often see false positives, as well as women who unknowingly have tumors that produce hCG. 

Main causes of false positive tests 

Changes in test results generally occur when the test has expired, and therefore it is very important to confirm the expiry test before using it. If the expiry date is correct, other reasons for a false positive may include: 

1. Test done incorrectly

Before using a home pregnancy test, be sure to read the package instructions. Pay attention to the time necessary to read the result, as some test results may change after a set time. 

In addition, before using the test, you should cleanse the genital area with water, as some soaps or creams can impact the test results, and show a false positive or false negative. 

Learn more about how to take a home pregnancy test. 

2. Chemical pregnancy 

This type of pregnancy occurs when the egg is fertilized, but the embryo is unable to implant into the uterus. In these cases, the body will start to produce hCG hormone, and therefore it will be detected in the urine or the blood. However, because the embryo fails to implant, it will eventually lead to a miscarriage, which can be confused for a late period. 

3. Use of some medications

Some medications used for fertility treatments contain high quantities of hCG. This hormone is what is identified in pregnancy tests, which can lead to false positive soon after a treatment. 

Other more common medications, like anticonvulsants, diuretics or tranquilizers can also cause changes to the final result. Therefore, it is important to read your medication pamphlets before completing a home pregnancy test, or to complete a blood test in the lab for more accurate results.

4. Health problems  

Although this is more rare, a false positive can also occur with some illnesses. This can happen, for example, with tumors that produce hormones, like breast cancer or ovarian cancer. 

Read about causes of false negative pregnancy tests. 

How to prevent a false positive 

To prevent a false positive result, it is important to follow the instructions as outlined in the pregnancy test package. After the test, you should also: 

• Repeat the test 3 to 5 days later 
• Do not reference your results after the indicated time 
• Book an appointment with your doctor within the next month 

The best way to prevent changes to your result is to complete an hCG blood test, as there is less room for error with this test method. Read more about the advantages and disadvantages of different pregnancy tests. In addition, before the test, the doctor will be able to evaluate whether there are any medications or health circumstances that can alter the results. 

Health

Sexual & Reproductive Health

It's rare, but it does happen.

While false positive pregnancy test results are rare, they do happen. Learn the potential causes and what to do when you get a false positive.

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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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:

1. Severe pain in your abdomen or pelvis while pregnant
2. Vaginal bleeding (that’s out of the ordinary)
3. Severe dizziness, lightheadedness, or fainting
4. Shoulder pain
5. An urge to poop
6. 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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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

7. Mayo Clinic, Ectopic Pregnancy

8. Mayo Clinic, Ectopic Pregnancy

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