At-home pregnancy tests work by measuring the amount of HCG (Human chorionic gonadotropin) hormone in your urine. This hormone is produced by the placenta after a fertilized egg attaches to the lining of the uterus. While not common, a false-positive pregnancy test can happen when the test indicates a positive result even when you are not pregnant.
False-positive pregnancy tests can be caused by many issues, most of which are not cause for concern. Here’s what you should know about getting a positive pregnancy test when you aren’t actually pregnant.
Potential Reasons for a False Pregnancy Test
False-positive results can be caused by anything from evaporation lines and certain prescription drugs to chemical pregnancy or even a defective test. The following are the most common reasons for a false-positive.
1. Evaporation Lines
Most pregnancy tests require that you read the results within a specific period of time. If you wait too long to read the results, urine on the test strip can evaporate. This can make the result look like two lines (positive) instead of one line (negative). With most tests, you must wait at least three to five minutes before you read the results. After 10 to 30 minutes, the results will no longer be accurate.
2. Chemical Pregnancy
A chemical pregnancy is also referred to as a very early miscarriage. This happens when a fertilized egg can’t implant and grow very early in the pregnancy for any number of reasons. Doctors believe that chemical pregnancies are very common and that most miscarriages happen before a woman knows she’s pregnant.
Because an ectopic pregnancy produces the pregnancy hCG hormone, it will normally result in a positive pregnancy test.
Some medications can interfere with a pregnancy test and cause a false-positive pregnancy test. The most common is fertility medication. If you are getting hCG injections to help you ovulate, a pregnancy test that detects hCG may not be effective.
There are also other medications that can interfere with the accuracy of a pregnancy test such as:
- Anti-anxiety medication
4. Medical Conditions
Some medical conditions can cause elevated hCG levels, even when you are not pregnant. The most common are:
- Ovarian cysts
- Urinary tract infection
- Disorders of the pituitary gland and disorders affecting hormone levels
- Kidney disease
- Cancer of the breast, ovary, bladder, lung, colon, kidney, liver, or stomach
Having one or more fibroids won’t result in a false-pregnancy test, but they can mimic some early pregnancy signs, such as frequent urination and pelvic pressure.
5. Molar Pregnancy
A molar pregnancy happens when there are genetic problems at conception, resulting in an abnormal pregnancy or uterine tumor. There are two types of molar pregnancies.
- The first is a complete molar type in which the mother’s chromosomes are absent in the tissue and there is no placenta or embryo.
- In a partial molar pregnancy, there is a placenta and embryo but it has two chromosomes sets from the father and one set from the mother.
Molar pregnancies are not viable and the tissue will develop into a mass of fluid-filled sacs. This type of pregnancy must be treated with a dilation and curettage (D&C) procedure. Even though it isn’t viable, a molar pregnancy can still cause elevated hCG that can be detected on a pregnancy test.
6. Previous Abortion or Miscarriage
If you have had an abortion or miscarriage recently, a home pregnancy test may not be reliable. This is because your hCG levels rapidly increase after a fertilized egg is implanted in the uterine lining.
During pregnancy, hCG levels double every few days before reaching a peak at 10 weeks. Hormone levels do decrease when a pregnancy ends, but this is a much slower process. The hCG hormone can even remain in detectable levels in your urine and blood for up to six weeks after a pregnancy is lost. Taking a pregnancy test during this time can cause a false-positive result.
In the case of miscarriage, it’s also possible for some tissue from the pregnancy to remain in the uterus. This tissue will continue producing the pregnancy hormone. Sometimes a D&C procedure is necessary to remove the tissue.
7. Expired Pregnancy Test
Make sure the test you are using is not expired. The chemical on the test strip used to detect hCG may not work reliably when it’s too old.
When used correctly, a home pregnancy test is about 99 percent accurate. Still, there are many reasons you can get a false-positive result. Always use the pregnancy test as directed and speak with your gynecologist or obstetrician about the accuracy of these tests if you believe any of the above reasons may apply to you. Your doctor can order blood tests to confirm or deny a pregnancy.