Although it’s rare, a false positive pregnancy test result can happen. For some women, this result can be disappointing, especially if they were hoping to become pregnant. A false positive test result—a positive human chorionic gonadotropin (hCG) test result when you are not pregnant—can occur for a variety of reasons.
Most over-the-counter pregnancy tests measure the levels of hCG in a woman’s urine. A urine pregnancy test is usually only positive in a woman who is pregnant and producing enough hCG to be detected by the test. A quantitative hCG test, which detects the level of hCG in the blood, can determine the exact age of the fetus and diagnose abnormal pregnancies. Blood tests can detect hCG at lower levels than urine tests, and therefore can detect a pregnancy earlier.
Of course, it is possible for a woman to become pregnant, get a positive test result, and then miscarry. This is known as a chemical pregnancy. A positive hCG test result does not necessarily indicate a viable pregnancy. If an at-home pregnancy test shows a positive result, it is important to have a healthcare professional confirm the result with a blood test; after that, it is important that you continue to see your healthcare practitioner on a regular basis so that he or she can help you maximize your chances of maintaining a healthy pregnancy.
Causes of a False Positive Pregnancy Test
There are many different reasons why a pregnancy test may yield false positive results.
Improper use of pregnancy test. One of the most common reasons at-home urine tests return false positives is that they were not done properly, usually because the instructions were misunderstood. It is important to read the manufacturer’s instructions carefully before taking a pregnancy test. Every test measures different levels of hCG.
Evaporation lines. You must read pregnancy test results within the time frame specified on the test. It is not at all unusual for evaporation lines to appear on the test strip as your urine dries. If you read the results after the recommended time has elapsed, you may confuse the appearance of urine evaporation lines with a faintly positive test line.
Expired pregnancy test. Always check the expiration date of the test before use. An expired test can yield false results.
Testing too early. According to fertility experts, up to 70 percent of conceptions end in miscarriage. You may experience a chemical pregnancy without ever realizing it if the miscarriage occurs soon after conception and your menstrual period arrives on time. Testing too early is common among women who are trying to conceive and are anxious to know whether they have. While early-detection pregnancy tests can show a positive result just days after ovulation, it is advisable to wait until after your first missed period to test. This can save you and your partner from the potential heartbreak of finding out that you are not expecting as you had hoped.
- Certain prescription medications can alter hCG test results. These include drugs used as part of fertility therapies, specifically drugs containing hCG. Other fertility therapies (such as clomiphene citrate and progesterone) and hormonal contraceptives should not affect the results. Medications that can affect hCG levels are:
- hCG injections
- Anti-Parkinson’s medicines
- Some tranquilizers
- Some diuretics
Contaminated urine sample. Another reason for a false positive test result is the presence of impurities in the urine sample, such as soap, detergent, or blood. Some pregnancy test kits come with cups. Washing the cup may inadvertently contaminate the urine sample if you do not wash out all the soap. If you do wash the cup, it’s crucial that you remove all soap or detergent before testing. It’s also important to know that these cups are generally sterile and don’t need to be washed before using.
Another potential contaminant is blood, which may be present in urine for reasons other than your menstrual cycle. Kidney disease and bladder infections, for instance, can cause hematuria. When this happens, it’s possible that the test will yield false positive results.
Medical conditions. Ectopic pregnancy, ovarian cysts, menopause, and other rare medical conditions can produce misleading results. A trophoblastic tumor is one example of a rare medical condition (only 3,000 cases occur in the United States each year) that can cause a false positive result. Trophoblastic tumors form when something goes wrong with the fertilization of an egg by the sperm. Instead of growing into a baby, the egg grows abnormally and becomes a tumor. These tumors develop in the reproductive tract and produce hCG.
Recent pregnancy. You may get a false positive result if you have recently been pregnant (even if you did not carry to full term).
How to Avoid a False Positive Test Result
Fortunately, false-positive pregnancy test results are rare. Most experts recommend taking two at-home pregnancy tests before contacting your healthcare provider to confirm that you are pregnant. Trying different brands of tests can reduce the likelihood that you have misread the results.
Most at-home pregnancy test kits are 99 percent effective. If you have followed the directions correctly, the odds are that your positive result is accurate and you are pregnant. However, it is important to remember that a positive result can be misleading. Also, when you take the test can make a huge difference in the result. It may be best to wait until after you have missed a period to take the test, even though you may be anxious to find out whether you are pregnant. Be sure to read the test’s directions thoroughly before use. Performing the test in the morning provides the best results, as hCG levels are highest at this time. If you are not sure about the results of your at-home pregnancy test, consider visiting the manufacturer’s web site or calling the manufacturer directly. Ultimately, however, you must visit your healthcare provider for an examination and quantitative blood testing to confirm your pregnancy.