Home pregnancy tests are typically accurate. However, in some instances, a test may produce a false-positive result.

Although many home pregnancy tests claim 99% accuracy, they may provide false results. A false positive is a test result that indicates the user is pregnant when they are not.

Several factors can lead to a false reading. This article discusses some of the potential causes of a person getting a false-positive pregnancy test reading.

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Home urine pregnancy tests work by detecting a hormone called human chorionic gonadotropin (hCG) in the urine. In early pregnancy, cells that will later form part of the placenta produce hCG. This hormone helps the body maintain a pregnancy.

Home pregnancy tests vary depending on the brand, and each one will have detailed instructions to follow for accuracy.

Some kits require a person to place a test stick in their urine stream. Others may use a urine collection cup. A person will then dip a pregnancy stick into this cup or retrieve urine with a dropper to place on the test stick.

The way the test reveals a positive or negative result also depends on the brand and can include:

  • a plus or minus sign
  • the terms “pregnant” or “not pregnant”
  • one or two indicator lines

Importantly, a person taking a pregnancy test should be aware that even a faint plus sign or a line indicates a positive result.

However, not all positive pregnancy tests mean the person is pregnant. There are some occasions when a pregnancy test can give a false-positive reading.

People should speak with their doctor for further evaluation if they receive a positive pregnancy test.

Here are five possible situations that could cause a person to have a false-positive result from a home pregnancy test.

When taking a home urine pregnancy test, it is important for people to follow the test’s instructions.

Most tests will ask the user to read the result within 4–5 minutes of taking the test and no longer than 10–30 minutes afterward.

Non-digital urine pregnancy tests will show lines or symbols to indicate whether a person is pregnant or not.

If a person reads the pregnancy test after the recommended time has elapsed, it may result in what looks like a positive test result. However, in most cases, this is not a positive pregnancy test but an evaporation line left by the urine after the recommended time for reading the test.

A person taking a home urine pregnancy test very soon after an abortion or miscarriage can be another reason for a false-positive result.

Once a fertilized egg implants into the uterine wall, the body secretes hCG. Typically, hCG may start to decline as soon as 5 days after a medical abortion but may remain at a readable level for longer. People taking a pregnancy test during this time can result in a false-positive test.

People may also experience an incomplete miscarriage. This condition means that there may be tissue from the pregnancy in the uterus that continues producing hCG. If this occurs, a person will need surgery to remove any remaining pregnancy tissue.

A molar pregnancy, or hydatidiform mole, is a condition that causes a uterine tumor to grow.

Instead of a normal placenta and embryo, the placental tissue develops abnormally. This can result in a cluster of cells, or a mass of small fluid-filled sacs forming, which a medical professional must remove by performing a dilation and curettage procedure, also called a D&C.

In a complete molar pregnancy, there is no embryo or placental tissue present in the growth. Conversely, in partial molar pregnancy, there may be some fetal tissue in the molar growth.

During a molar pregnancy, a person’s hCG levels will increase. This may lead to a positive pregnancy test.

At times, a condition called persistent gestational trophoblastic disease can occur.

This results from the continued growth of the products of a molar conception within the uterus. It can be a serious medical condition requiring chemotherapy or surgery to remove the womb.

Certain medications may cause false-positive pregnancy tests by raising a person’s hormone levels in their blood and urine.

Certain infertility and weight loss treatments may contain synthetic hCG and cause false-positive results in blood tests. Other drugs, such as aspirin, carbamazepine, and methadone may also result in false-positive tests.

Alongside gestational trophoblastic diseases, which includes molar pregnancy, other medical conditions can cause a person’s hCG levels to rise, even when they are not pregnant. These include:

  • disorders affecting the pituitary gland and hormone levels
  • cancers of the ovary, bladder, kidney, liver, lung, colon, breast, and stomach
  • phantom hCG, where antibodies interfere with the testing kit
  • ovarian cysts that produce hCG
  • kidney disease or urinary tract infections

Phantom hCG occurs when certain antibodies bind with the molecules in the pregnancy testing kit. The interaction causes the testing kit to signal that hCG levels are higher than they are.

Home pregnancy tests can be up to 99% accurate when used correctly.

The amount of hCG present in a person’s urine gradually increases following fertilization. Due to this, tests will typically be more accurate following a person’s first or second missed period.

However, in around 10% of pregnant people, hCG levels only begin to rise the first day after a missed period. In these instances, a test is unlikely to be accurate if carried out at this time.

Not using the test as advised or checking the results later than the recommended time can lead to false-positive results.

If someone has a positive pregnancy test, it is important to notify their doctor, as they will need further evaluation and testing to confirm and monitor their pregnancy.

If someone believes that they may be pregnant, but their home pregnancy test results are negative, they should speak with a doctor for further guidance.

Home pregnancy tests can be up to 99% accurate. However, in some instances, they may produce a false-positive result.

Incorrect test usage, previous abortions and miscarriages, and some medications may lead to a false-positive pregnancy test result.

People should always see their doctor after a positive pregnancy test. Further tests can assess the reasons for a positive result and confirm whether a person is pregnant or not.