Home pregnancy tests are a common tool used to find out if you re expecting. Most at-home pregnancy tests are dipsticks. They are are placed into a urine stream. The stick is then able to detect human chorionic gonadotropin (hCG). This is a hormone produced during early pregnancy.
Some pregnancy tests detect hCG very early in pregnancy. Reputable home pregnancy tests can be highly accurate, but they aren’t foolproof.
False positives and negatives can occur for a variety of reasons. It’s also important to remember that once you have a positive pregnancy test, see your doctor to begin early prenatal care.
Read on to learn more about false positives on home pregnancy tests.
1. Chemical pregnancy
It’s possible to have a positive pregnancy test even if you aren’t technically pregnant. This is called a false positive.
It’s sometimes caused by a chemical pregnancy. A chemical pregnancy occurs if a fertilized egg, known as the embryo, is unable to implant, or grow, very early on. This can happen for a wide range of reasons.
Early pregnancy loss doesn’t occur because of anything the woman has done. It may be the result of issues within the uterus, such as:
- Scar tissue
- A congenital uterine anomaly that causes an irregular-shaped uterus
Low amounts of certain hormones, such as progesterone, can reduce the likelihood of implantation and embryo growth.
Some causes of chemical pregnancy are unknown.
Chemical pregnancies are thought to be very common, but they typically go undetected if a pregnancy test isn’t taken. These early test results, when wrong, can be emotionally draining.
For that reason, it’s recommended you wait to use an at-home pregnancy test until one week after you expected your period to start.
Sometimes a fertilized egg can implant itself outside of the main cavity of the uterus. This causes an ectopic pregnancy to occur.
Ectopic pregnancies usually happen if a fertilized egg gets stuck in a fallopian tube during its journey to the uterus. This type of ectopic pregnancy is also known as a tubal pregnancy.
The following may lead to an ectopic pregnancy:
- scar tissue or inflammation in the fallopian tube
- misshapen fallopian tube
- a history of past uterine infections
Ectopic pregnancies can also occur in the cervix, ovary, or abdominal cavity.
An ectopic pregnancy can’t continue to become a normal pregnancy. The embryo isn’t viable, because there’s no place for it to grow or thrive outside of the uterus.
The embryo will still produce hCG, even though it has implanted in the wrong place. That can cause a false-positive reading on an at-home pregnancy test.
Ectopic pregnancies are medical emergencies. Ectopic pregnancies can be damaging to the woman if left untreated. Extreme blood loss or loss of the reproductive organs can occur.
Symptoms of ectopic pregnancy include:
- nausea and sore breasts, which are also symptoms of a normal pregnancy
- sharp waves of pain in the abdomen, pelvis, shoulder, or neck
- severe pain on one side of the abdomen
- light to heavy vaginal spotting or bleeding
- dizziness or fainting
- pressure on your rectum
Seek immediate medical help if you suspect you have an ectopic pregnancy.
3. Recent miscarriage or abortion
You may continue to test positive for pregnancy following the loss of a pregnancy, either through miscarriage or abortion.
During pregnancy, hCG levels continue to rise as the placenta grows, doubling every few days and peaking at around 10 weeks. When a pregnancy ends, hCG levels begin to recede, but it’s a slow process.
The hormone can remain in your blood and urine for up to six weeks following the end of the pregnancy. It’s possible to have a false-positive test until your hCG levels return to their prepregnancy state.
If the miscarriage was spontaneous, it’s also possible that not all of the pregnancy-related tissue was eliminated. This will cause hCG levels to remain elevated.
When this occurs, a minor surgical procedure called a dilation and curettage (D and C) is often required to remove the tissue.
4. User error
Like most things in life, at-home pregnancy tests aren’t foolproof. It’s important to follow package directions exactly. Check the expiration date before using the test.
Even with these safeguards, user error can occur. One of the most common mistakes is taking the test too early during your cycle. This can cause either a false negative or a false positive.
It’s also important to use the test when your urine isn’t diluted excessively with water. Use the test when your urine is very concentrated, like when you first wake up in the morning.
Leaving the dipstick in your urine stream for the exact amount of time allotted is also important. Consider setting a timer on a stopwatch or your phone. That can help you track how long the dipstick has been in your urine stream.
You’ll want to use a timer again while you wait for your results. Checking your results during the result time frame is also important.
5. Evaporation lines
Sometimes an evaporation line can be mistaken for a positive pregnancy test. Some at-home tests show two lines when hCG is detected and one line when hCG isn’t detected.
The lines are usually a bright color, such as pink, red, or blue. Sometimes, a faint-colored second line will appear. This line may represent an early pregnancy or it may be an evaporation line.
It’s probably an evaporation line if the line is completely colorless.
Evaporation lines may show up on a test you view after your urine has evaporated completely. Sometimes they’re caused by hormonal levels that don’t represent pregnancy.
The best way to avoid being confused by an evaporation line is to follow the test’s timing directions exactly as they’re given.