Causes of a False Positive Pregnancy Test

Maybe you can’t wait to have a baby. Or maybe that’s the last thing you’re hoping for.

Either way, if you think you might be pregnant, you need to know for sure. But what are the odds a positive pregnancy test might be wrong?

“Home urine pregnancy tests are pretty reliable,” says Ob/Gyn Jonathan Emery, MD. “But there are some reasons you might get a false-positive result.”

Dr. Emery explains when and why a pregnancy test might give a false positive — and what you can do to make sure the stick doesn’t lie.

How do pregnancy tests work?

When you get pregnant, your body produces a hormone called human chorionic gonadotropin, or hCG. Home pregnancy tests look for that hormone in your urine. If hCG is present, you should get a positive test result when you pee on a stick.

But hCG levels start out very low and increase over time. If you take the test too soon after conceiving, it might say you’re not pregnant when you really are.

In other words, timing can lead to a false negative. But what about a false positive?

Chances of a false positive pregnancy test

False positives aren’t super common, Dr. Emery says. But they’re not impossible. Some potential causes include:

Early miscarriage or chemical pregnancy

You took a pregnancy test and got two lines. (Positive!) But a few days later, your period arrived in force. What gives? The most common reason this happens is an early pregnancy loss, also known as a chemical pregnancy. In this case, the test was accurate — there was a pregnancy, but it wasn’t a viable one, Dr. Emery explains.

“It’s not technically false since a very early pregnancy did occur,” he says. “But this is the most common reason that a pregnancy test might appear to have been false.”

Fertility medications

“A lot of fertility treatments involve taking hCG injections,” Dr. Emery says. If you’ve been taking fertility medications, that hCG might still be floating around your system.

That could trigger a positive pregnancy test, even if you’re not pregnant. To avoid that mix-up, wait at least two weeks after your last hCG injection to take a home pregnancy test, he says.

Recent pregnancy

If you were recently pregnant, you might still have leftover hCG in your system. After childbirth, miscarriage or treatment for ectopic pregnancy, the hormone can remain in your body for up to four to six weeks, Dr. Emery says. “That could lead to a positive pregnancy test when your body just hasn’t cleared the hCG yet.”

User error

Home pregnancy tests aren’t especially hard to use. But you still have to pay attention since a mistake can lead to incorrect results.

If you wait too long to read the results, for example, or use more drops of urine than the test calls for? You might want to take the answer with a grain of salt. “If you don’t follow all the instructions, any results — positive or negative — could be false,” Dr. Emery says.

Pregnancy test accuracy: How to get results you can trust

Luckily, false positives are rare. And there are steps you can take to make sure your home pregnancy test gives you results you can trust.

• Time it right. Don’t take a test too early. It’s more likely that you’ll get a false negative — or that the test will detect a chemical pregnancy that isn’t able to progress. Dr. Emery recommends waiting until the day of your missed period, or a few days later, to take the test. “Timing is important. The test is most accurate if you wait at least until the day of your expected period,” he says.
• Follow the directions. Make sure your test isn’t expired. Read the directions before you start. And follow the steps exactly to avoid a stressful false result.
• Repeat it. If you got a positive home pregnancy test, you might be eager to get a blood test to confirm the result. Blood tests are more accurate, so that’s certainly an option. But it isn’t always necessary, Dr. Emery says. “If you get a positive result from a urine test, then repeat the test in three to five days. If it’s still positive, you can trust the result,” he says.

Home pregnancy tests are inexpensive, private and quite reliable, Dr. Emery adds. “People often don’t believe what they’re seeing. But if you’ve used the test correctly and done it at the right time, it’s probably true.”