Home pregnancy tests are a common tool used to find out if you’re expecting. Most at-home pregnancy tests are dip sticks, which are placed into the urine stream. The stick is then able to detect human chorionic gonadotropin (hCG), 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.

It’s possible to have a positive pregnancy test even if you aren’t technically pregnant. This is called a false positive, and is 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 fibroids, scar tissue, or a congenital uterine anomaly which causes the uterus to have an irregular shape.

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 is not taken. These early test results, when wrong, can be emotionally draining. For that reason, it is recommended that you wait to use an at-home pregnancy test until one week after you expected your period to start.

Read more: 17 Early pregnancy symptoms »

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 can happen if the tube has scar tissue, inflammation, is misshapen, or if there is a history of past uterine infections. This type of ectopic pregnancy is also known as a tubal pregnancy.

Ectopic pregnancies can also occur in the cervix, ovary, or abdominal cavity. An ectopic pregnancy cannot continue to become a normal pregnancy. The embryo is not 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, causing extreme blood loss or loss of the reproductive organs. 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 an ectopic pregnancy.

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 loss of your 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 is often required to remove the tissue.

Like most things in life, at-home pregnancy tests aren’t foolproof. It’s important to follow package directions exactly and to check the expiration date prior to use. 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 is not diluted excessively with water and is very concentrated, such as 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. Considering 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.

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 is not 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.

Learn more: Faint positive home pregnancy test: Am I pregnant? »

Evaporation lines may show up on a test you view after your urine has evaporated completely. Sometimes they’re caused by hormonal levels which 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 are given.

If you’re trying to become pregnant under a doctor’s care, you may be taking fertility medications. One of these is the synthetic hCG trigger shot, sold under the brand names Novarel, Pregnyl, Ovidrel, and Profasi. The hCG shot helps follicles release mature eggs. It may cause a false-positive reading on an at-home pregnancy test, particularly if the test is taken too early.

Other medications can also cause false-positive pregnancy tests. They include but are not limited to:

Rarely, certain medical conditions can cause a home pregnancy test to give a false positive. These include:

  • a urinary tract infection
  • kidney disease that causes blood or white blood cells in the urine
  • ovarian cysts, particularly corpus luteum cysts
  • more serious diseases such as ovarian cancer
  • pituitary problems (very rarely)

A positive at-home pregnancy test result should always be followed up with a doctor’s appointment. Your doctor may give you a urine or blood test to confirm the results and monitor your hCG levels. You may also get a transvaginal ultrasound to look for the gestational sac as confirmation that the pregnancy is proceeding normally.

If you have received a false positive, your doctor’s visit will determine that. It might be an incredible relief to find out you are not pregnant. But if you were excited by your early results, it can be very upsetting. It’s important to remember that false positives do happen and are not an indication that you will never get pregnant.

If you’ve been trying unsuccessfully to get pregnant for some time, particularly if you are over the age of 35, consider working with an infertility specialist. There are also support groups where you can find inspiration and obtain knowledge from women who have been through the same thing. Working one-on-one with a therapist, clergy member, family member, or trusted friend can also be beneficial.