What Causes a False Positive on a Pregnancy Test?

Pregnancy tests give accurate results about 99% of the time, as long as they are used correctly. Still, incorrect results can happen. For example, a pregnancy test might indicate that you’re pregnant even if you aren't. This is called a false positive result.

Though rare, false positive pregnancy results do happen. This is usually due to user error. However, it can also occur due to factors like early pregnancy loss, certain medical conditions, and medications.

1. User Error

User error is the most common cause of false positive pregnancy test results, meaning that the pregnancy test was not used according to the instructions. The 99% accuracy rating of home pregnancy tests is based on tests used in a laboratory setting. This means the test was performed by technicians. Errors can occur when you use one yourself.

One common mistake is waiting too long to read test results. Tests are typically read within 10 minutes of taking the test. For some tests, a line on the test strip changes color to indicate a pregnancy. In this case, it’s possible for an “evaporation line” to appear if the test sits for longer than the directions say it should. Using home pregnancy tests that simply display results in words—for example, “Pregnant” or “Not Pregnant”—can help you avoid this error.

2. Chemical Pregnancy

False positive pregnancy results often occur because you are no longer pregnant even though conception happened recently. This has to do with the hormone human chorionic gonadotropin (hCG), which is released when you are pregnant. This is the hormone that both urine-based and blood-based pregnancy tests detect, even in small amounts.

If you experienced a very early pregnancy loss—often referred to as a chemical pregnancy—your body may have produced enough hCG for the test to register as positive even though you are no longer pregnant. Early losses and chemical pregnancies are estimated to impact about 15-20% of conceptions.

3. Miscarriage or Abortion

If you’ve recently had a miscarriage (spontaneous abortion) or an abortion, you may still have enough hCG in your blood for your pregnancy test to show a positive result. It can take 7-60 days for hCG levels in your blood to return to zero.

An “incomplete miscarriage" means that some pregnancy tissue remains in your uterus. This can also lead to continually elevated hCG levels and lead to a positive test result. Contact your OB-GYN or midwife if you have signs of a miscarriage or incomplete miscarriage, such as:

• Bleeding, which may be light or heavy
  
• A gush of fluid from your vagina
• Cramping
• Tissue passing through your vagina

It’s important to remove all pregnancy tissue from your body if any remains.

4. Medical Conditions

Though less common, certain medical conditions can cause hCG to be present in your urine or blood, even though you aren’t pregnant. These medical conditions include:

• Ectopic pregnancy: When a fertilized egg grows outside the uterus
• Molar pregnancy: When fluid-filled sacs grow in your uterus during pregnancy, instead of a placenta, which prevents a fetus from developing
• Certain cancers: Including myeloma (a form of blood cancer), stomach, lung, liver, bladder, breast, colon, pancreatic, and uterine cancer
• Having heterophile antibodies: Antibodies produced after exposure to certain animal products
• Rheumatoid factors: Proteins produced by your immune system that sometimes attack healthy tissue
• Immunoglobin A (IgA) deficiency: Low levels or the absence of the antibody IgA

People who are in perimenopause (the transition years leading to menopause) or menopause are also more likely to experience a false positive pregnancy test due to hormone changes.

5. Medications

Certain medications can cause a false positive result. For example, you may get a positive pregnancy test result if you use fertility medications that contain hCG. You might use these medications to trigger ovulation for intrauterine insemination (IUI) or in vitro fertilization (IVF). Brand names include:

• Pregnyl
• Ovitrelle
• Predalon

There have also been some reports of antidepressants—particularly selective serotonin reuptake inhibitors (SSRIs)—giving false positive results on pregnancy tests. More research is needed in this area.

Always consult with your OB-GYN or midwife if you believe you took a pregnancy test that isn’t accurate. They can advise you on medications that might be affecting your results.

How Do Home Pregnancy Tests Work?

Home pregnancy tests detect human chorionic gonadotropin (hCG) in your urine. This hormone is secreted by your embryo beginning in early pregnancy. Within a week or so after you conceive, your body produces hCG.

The amount of hCG in your blood doubles each day. This is why detecting it in your urine becomes easier as the pregnancy progresses. Levels of hCG peak around 10 weeks after conception and stay stable throughout the rest of your pregnancy.

Multiple factors can affect the accuracy of home pregnancy tests, including:

• How you use them: User error is the most common cause of a false positive result.
• When you take them: The test might not detect a pregnancy if you take it very early, due to low levels of hCG at that point.
• Cycle differences: For example, a longer time between ovulation (when an ovary releases an egg for possible fertilization) and implantation (when the fertilized egg attaches to the uterine wall) might mean that you need to do a pregnancy test at a later point to detect pregnancy.
• Test brand: Some tests are more sensitive, so they may detect pregnancy earlier on.
  

How to Take a Home Pregnancy Test

Home pregnancy tests are usually highly accurate, but you have to use them correctly. Urine tests are typically the most accurate 1-2 weeks after a missed period. Blood tests performed at a provider's office usually detect pregnancy earlier.

Each brand of test has specific instructions, so always read the packet that comes with your test. Most brands recommend the following:

• Place the test strip under where you urinate so that you can catch urine on the strip. Alternatively, the test may ask you to urinate in a cup and then dip the test strip in the cup.
• Wait the allotted number of minutes for the test to work, usually about three minutes.
• Read the results, according to the directions. A positive result might appear as two lines, a “plus” sign, or words like “Pregnant” in the test window.

Do not use a test after its expiration date, and do not read the test after the amount of time noted on the instructions. Your first urine of the morning usually has the most hCG in it, so doing the test first thing in the morning can also help ensure accurate results.

What to Do After a Positive Pregnancy Test

You are most likely pregnant if you get a positive result from a pregnancy test. Do your best to make an appointment with a healthcare provider as soon as you get a positive result. Prenatal care is important for you and the developing fetus and helps support a healthy pregnancy.

Schedule an appointment even if you experience what you believe is a false positive pregnancy test. You may be pregnant, even if you think you aren’t. Your provider will also want to rule out other possible factors that may be causing the positive pregnancy test, such as an underlying medical condition. They may want to consider whether you’ve recently had a miscarriage and if any further steps need to be taken to ensure that you’ve fully cleared the pregnancy from your body.

A Quick Review

It’s not common to get a false positive on a pregnancy test, but it does happen. This is usually because the test was not used correctly. Other common causes include chemical pregnancies, miscarriages, certain medical conditions, and medications.

Always report a positive pregnancy test to your healthcare provider. They can help you understand if the test result is accurate and guide you through the next steps.