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Why False Positive Pregnancy Tests Can Happen?


When taking a home pregnancy test, there’s a small chance the results will be inaccurate. A false positive pregnancy test means your results showed positive, but you’re not pregnant. There are several reasons why you might have a false positive pregnancy test.

Here, we’ll discuss the accuracy of home pregnancy tests, why some people might have a false positive or false negative test, and when you should see a doctor after taking a home test.

How accurate are home tests?

Most manufacturers of home pregnancy tests say there's a 99% accuracy rate when used as directed. The tests work by measuring levels of human chorionic gonadotropin (hCG), a hormone produced to help support pregnancy. Some tests can detect this hormone very early in pregnancy.

Every test will contain instructions on how to take the test, but in general, it requires you to place the testing stick in a cup of urine, or by holding the stick in a urine stream. The test is placed on a clean flat surface and the results are ready in several minutes.

The plus sign or line might be hard to see, but even a faint reading can indicate a positive test. Usually, this means the pregnancy is still early, or the levels of hCG aren’t yet high enough to give a more definitive reading.

The accuracy of these tests depends on several factors:

  • Are you using the test correctly? Don’t use a test if it’s expired, and make sure you are waiting the correct amount of time before reading the result.
  • Are you taking the test at the right time? If you’re testing before your missed period, the test may not be able to pick up enough hCG. Taking a test with your first-morning urine can be the most accurate.
  • What type of test are you taking? Some brands are more sensitive than others and can detect hCG before your missed period.

What causes a false positive pregnancy test?

It’s not common, but there is a chance you may get a false positive pregnancy result. There are several reasons this could be.

Evaporation lines

If you let the pregnancy test sit too long and don’t read the results in the time indicated in the packaging instructions, you might actually get a false positive result. This is because evaporated urine can leave a faint line that appears to be a positive result.

Taking fertility medications

Some medications, such as ones used in fertility treatments, can contain the hormone hCG. Taking a home pregnancy test while on these medications might give a false positive. Similarly, if you have just stopped taking fertility medications, you could have enough hCG in your body to give a false positive result, and you aren’t actually pregnant.

Chemical pregnancy

Also known as an early miscarriage, a chemical pregnancy means there was a pregnancy, but it wasn’t viable. You might see a positive result on your pregnancy test, but then your period comes a few days later. This doesn’t mean it was a false positive since you really were pregnant. But it is a situation that appears to cause a positive result.

Recent miscarriage

If you have had a miscarriage your levels of hCG can still be elevated in your body for 4 to 6 weeks. If you take a pregnancy test during this time, you might see a positive pregnancy test. However, it can mean your hCG levels might not have lowered to the level it was before you were pregnant.

Ectopic pregnancy

If a fertilized egg implants outside of the uterus such as in the fallopian tube, it’s known as an ectopic pregnancy. This is a serious medical event that requires care immediately. The fertilized egg, or embryo, is still producing hCG, and therefore, you’ll still see a positive pregnancy test. But this is a type of pregnancy that isn’t viable because it’s unable to grow outside of the uterus.

Certain medical conditions

Some medical conditions might cause a falsely positive pregnancy test, including:

  • Certain tumors
  • Kidney disease
  • Bladder infections
  • Chemotherapy

These conditions might cause your hCG levels to increase in your body even though you’re not pregnant.

When to see a doctor after taking a pregnancy test

Because it’s still rare to get a false positive on your pregnancy test, notify your healthcare provider if you get a positive test. You can get a blood test at the clinic if you’re unsure of your results. Your healthcare provider can confirm your results.

In order to have the healthiest pregnancy possible, it’s important to begin prenatal care right away. You’ll typically have an ultrasound at your first prenatal visit in order to figure out your due date.

Can you trust a negative test result?

A negative test result means there’s no hCG detected in the test. Rarely, false negatives can occur, due to several reasons, such as:

  • You tested too early.
  • Your urine is too diluted and there isn’t enough hCG to test.
  • You took the test incorrectly.

If you get a negative test but believe you could be pregnant, wait until you have a missed period, or 14 days after you think you conceived, to take another one.

Conclusion

It’s not common to have a false positive pregnancy test, but it is possible. There are different reasons why this can occur including taking the test incorrectly, certain medical conditions, or taking fertility medications. Talk with your healthcare provider if you have concerns about your pregnancy test results or want to confirm you are pregnant.

Key takeaways

Home pregnancy tests are 99% accurate when used correctly.

User error, medical conditions, and certain medications can cause false positive results.

See your healthcare provider if you need to have your pregnancy confirmed.

Resources:

Office on Women's Health. Pregnancy Tests.

StatPearls Publishing. Human Chorionic Gonadotropin.

Cleveland Clinic. Causes of a False Positive Pregnancy Test.

American Pregnancy Association. What is HCG?

The American journal of case reports. A Case of a False-Positive Urine Pregnancy Test and Delayed Diagnosis of Obstructive Pyelonephritis.

Obstetrics and Gynecology. Reducing False-Positive Pregnancy Tests in Cancer Patients.

Mayo Clinic. Ectopic pregnancy.

Mayo Clinic. Home pregnancy tests: Can you trust the results?

American Pregnancy Association. Pregnancy Tests.

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