What Causes a False Positive Pregnancy Test?

positive pregnancy testFew things can make your stomach flip like a positive pregnancy test. And although at-home, over-the-counter tests are 97 percent accurate -- which is pretty darn good -- that still means there's a 3 percent chance it's wrong. And if you're part of that 3 percent, it's a huge deal!

You and your partner could be doing cartwheels (or the exact opposite) for a false alarm. And it turns out there are a whole slew of things that can cause a false positive pregnancy test, so make sure to rule these out first before you start celebrating, or freaking out ... or doing some combination of both.

  1. A chemical pregnancy. This type of early pregnancy loss is the most common reason for a false positive. "In this case, the woman has in fact conceived and the pregnancy was able to implant and then be detected in the urine of mom," explains Danielle Lane, MD, medical director or the Lane Fertility Institute. "But the pregnancy is not able to move forward. So the woman has a positive test and then, in future days, a negative test after she miscarries." In most cases, these early miscarriages are the result of genetically abnormal pregnancies.
  2. You didn't follow directions. "The most common misuse of a pregnancy test is letting the test sit for too long, resulting in a visual evaporation line that looks like a positive reading," says Jane Frederick, MD, a fertility doctor in Los Angeles and co-medical director at HRC Fertility.
  3. The pregnancy test is defective. Be sure to check expiration dates before you buy -- they do matter. In some rare cases, however, the test itself is actually defective. Since there is no way to tell from the actual test that it's flawed, your best bet is to just take another test -- preferably a different brand -- or if you're using the same brand, try a different store, since defective tests could come in batches.

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  4. You're taking medication. Certain medications can result in a false positive, particularly if you're undergoing fertility treatments. "A rise in HCG medication may result in a positive pregnancy test," Dr. Frederick explains. "This is known as 'phantom HCG.'" Other non-fertility medications that can cause a false positive include: methadone (for pain relief), chlordiazepoxide (for anxiety), and promethazine (for nausea, allergies, anxiety, or sleeplessness).
  5. You have an illness. Certain rare diseases such as trophoblastic disease (tumor growth inside the uterus) and islet cell tumors (tumors in the pancreas) can also cause false positives. But they are rare: less than .1 percent of women get trophoblastic disease, and only 1,000 new cases of islet cell tumors occur in the US per year. Still, they're worth mentioning, particularly if these conditions run in your family.

Whatever the cause, if you're staring at what you suspect is a false positive reading, it won't hurt to take another test -- or go for a (more accurate) blood test at your doctor's office.

Did you ever have a false positive pregnancy test?


Image © IAN HOOTON/Science Photo Library/Corbis

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