At-home pregnancy tests are surprisingly accurate, but it is still possible to get a false negative result or even a false positive pregnancy test result.
There are several different reasons why pregnancy test results may not be entirely accurate — here are the 7 most common causes of a false positive.
How Do Pregnancy Tests Work?
Most women use at-home pregnancy tests if they think they might be pregnant.
These tests are usually urine pregnancy tests where the user will either urinate directly on the test stick, or urinate into a cup and then dip the test stick into it.
At-home pregnancy tests work by detecting levels of a hormone called hCG, or human chorionic gonadotropin.
The hCG hormone is considered the pregnancy hormone because its levels immediately begin to significantly elevate at the very start of pregnancy.
If the pregnancy test detects enough levels of hCG in your urine, the test will be positive, indicating you are pregnant.
If you get a positive test at home, be sure to follow up with your OB-GYN or other women’s healthcare provider to confirm your results with a blood test.
Can You Get a False Positive Pregnancy Test?
Whether you choose to take an at-home pregnancy test or go to your doctor for bloodwork, pregnancy tests are considered highly accurate — 99%, in fact.
However, it is possible to receive a false negative pregnancy test or, less commonly, a false-positive result, which either mistakenly indicates a pregnancy, or indicates a pregnancy that is not viable.
What Can Cause a False Positive Pregnancy Test?
There are seven primary causes for a false-positive pregnancy test: chemical pregnancy, recent pregnancy loss, evaporation lines, medical conditions, ectopic pregnancy, user error, and the use of certain medications.
One common cause of false positive pregnancy tests is chemical pregnancy. A chemical pregnancy occurs during ovulation when an egg is fertilized by a sperm, creating an embryo, but is unable to grow and develop properly to be viable past the first month or so.
Chemical pregnancies are miscarriages that occur during the first five weeks of pregnancy, often before a woman even knows that she is pregnant.
The body starts to produce hCG in response to the embryo’s development, but these levels begin to drop following the miscarriage.
If a woman takes the pregnancy test before the miscarriage or even shortly after and elevated levels of hCG are likely still detectable in her urine stream, she may receive a false positive result.
Recent Pregnancy Loss
Whether it is an early pregnancy loss such as an early miscarriage or a later pregnancy loss such as a stillbirth, a woman’s hCG levels remain elevated for up to six weeks after the end of a pregnancy.
Women who have an abortion will also continue to have a higher amount of hCG in their system following the procedure.
If a woman takes a pregnancy test within the first six weeks after experiencing pregnancy loss, she may receive a possible false positive test result.
Miscarriages that are spontaneous may not result in the complete elimination of pregnancy-related tissue, which can keep hCG levels up.
The appearance of evaporation lines can cause people to interpret the results of an at-home pregnancy test incorrectly.
Many at-home pregnancy tests show one line when hCG is not detected (a negative test result) and two lines when hCG is detected (a positive result).
The second line typically appears as a bright color, making the test easier to read.
However, a second line may also appear due to evaporation, particularly if the line appears colorless.
It’s important to read the test instructions carefully and check the results after the specified amount of time. Waiting too long can result in the appearance of a second line and a false positive result.
Although rare, some medical conditions may also cause a person to receive a false-positive test result.
These conditions can result in the disruption of a woman’s hormones, which can skew the results.
Medical conditions that may result in a false positive pregnancy test include:
- Molar pregnancy
- Hydroxychloroquine (HCQ) injections
- Urinary tract infection
- Certain types of ovarian cancer
- Rare antibodies
- Kidney disease
- Pituitary problems
An ectopic pregnancy is a serious medical condition in which a fertilized egg implants outside of the main cavity of the uterus, often in the fallopian tube.
When the body detects the embryo it will begin to produce hCG.
Though the woman is technically “pregnant,” this is considered a false positive result because the embryo will not survive.
If the embryo continues to develop outside of the uterus, the condition can be life-threatening to the woman because there is nowhere for the embryo to grow, causing internal rupture and bleeding.
Symptoms of an ectopic pregnancy include:
- Sharp, intense pain in the abdomen, pelvis, shoulder, or neck
- Heavy vaginal bleeding or consistent spotting
- Pressure on the rectum
- Dizziness or fainting
- Severe pain on one side of the abdomen
An ectopic pregnancy can cause extreme blood loss and damage to or loss of reproductive organs.
Women who think they may be experiencing an ectopic pregnancy should seek medical attention immediately.
At-home pregnancy tests are about 99 percent accurate when used correctly, but it is possible to get a false positive as a result of user error.
To get the most accurate result, women should follow the package instructions carefully and make sure that the test is not expired.
Results are most accurate when taken at least one week after a missed period.
Taking a test too early can result in a false negative or a false positive.
Pregnancy tests should be taken when hCG is most concentrated in the urine, typically early in the morning.
After taking the test, people should wait the directed amount of time before checking pregnancy test results.
Checking the results too early or too late can result in an inaccurate test result.
People who are trying to get pregnant with the help of a doctor should be aware that some fertility medications can also cause a false positive pregnancy test.
Many patients receive an hCG trigger shot, such as Novarel, Pregnyl, Ovidrel, or Profasi, during fertility treatments to trigger the release of mature eggs.
Taking a pregnancy test too early in the cycle while taking these medications can cause a false-positive result.
Other medications can also contribute to a false-positive result. These include:
- Anti-anxiety medications, like diazepam (Valium) or alprazolam (Xanax)
- Anticonvulsants, like phenobarbital or other barbiturates
- Diuretics, like furosemide (Lasix, Diuscreen)
- Antihistamines, including promethazine
- Methadone (Dolophine)
- Antipsychotics, such as clozapine or chlorpromazine
- Parkinson’s disease medications, including bromocriptine (Parlodel)
The Bottom Line
A false pregnancy test result can occur because of chemical pregnancy, recent pregnancy loss, evaporation lines, medical conditions, ectopic pregnancy, user error, and the use of certain medications.
The best way to avoid a false positive test is to use the test exactly as directed and pay attention to any symptoms you might experience or medications you might be using.
References and Sources:
Bridget Reed is a Tampa-based content development manager, writer, and editor at GR0; specializing in content related to varying fields including medicine, health, and small businesses. Bridget went to St. Petersburg College and majored in Management and Organizational Leadership.
Recent Publications: Body Acne 101: Prevention and Treatment, Stress Acne: Causes, Prevention, and Treatment, What are the Side Effects of Midol?