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Pregnancy Test Evaporation Lines: What Are They?

evaporation line pregnancy test

You might suspect you’re pregnant if you’ve missed a period or are experiencing morning sickness. Even if your instinct says you’re expecting, you’ll still have to confirm it with a pregnancy test.

You can pick up a home pregnancy test at your local drugstore. These tests are 97 to 99 percent accurate. But sometimes, the results are confusing.

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Some pregnancy tests involve two lines: a control line and a test line. The control line appears on every test, but the test line only appears if there are levels of the pregnancy hormone in your urine.

If you take a pregnancy test and see two lines, you may think you’re pregnant. But the appearance of two lines when using a home test doesn't necessarily mean you're pregnant. The second line could be an evaporation line.

Here’s why you might get an evaporation line on a pregnancy test.

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How does an at-home pregnancy test work?

An at-home pregnancy test is a simple way to find out if you're pregnant before seeing a doctor. When you schedule an appointment with your doctor to confirm a pregnancy, your doctor may take a urine or blood sample. A lab checks these samples for a hormone the body produces during a pregnancy, called human chorionic gonadotropin (hCG).

This hormone is released into the bloodstream once a fertilized egg implants in the uterus. The body produces a low level of hCG during early pregnancy. The level increases as a pregnancy progresses. At-home pregnancy tests are designed to detect this hormone.

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Typically, an at-home pregnancy test involves urinating on a test stick and checking the results minutes later. If your pregnancy test result only reveals one line (the control line), it often means you’re not pregnant. If your test results reveal the control line and the test line, this can indicate a pregnancy. Always check the test instructions for an evaporation line.  

What is an evaporation line on a pregnancy test?

Evaporation lines are common and can occur with any pregnancy test. An evaporation line is a line that appears in the results window of a pregnancy test as the urine dries. It can leave a faint, colorless line.

If you’re not familiar with evaporation lines, you might see this line and think you’re pregnant. This can result in disappointment when a doctor confirms a pregnancy hasn’t occurred.

You can’t control whether an evaporation line appears in your results window. But you can learn how to distinguish a positive test line from an evaporation line.

How to identify an evaporation line on a pregnancy test

Evaporation lines are common on pregnancy tests, but they don’t appear every time. It depends on the chemical makeup of each woman's urine. One of the best ways to avoid any confusion when using a home pregnancy test is to check your results within the reaction time. This is the window to receive an accurate result, and it varies by brand.

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Every home pregnancy test comes with instructions. Pregnancy tests are easy to use, so you might open a pregnancy test kit and take the test without reading the instructions. But if you want to avoid mistaking an evaporation line for a positive test line, you have to follow the instructions and check your results before the urine completely evaporates.

Some pregnancy tests have instructions to check results after two minutes. Others have instructions to check results after five minutes. The risk of a false positive is higher when you read your results after the reaction time.

How to avoid getting an evaporation line on a pregnancy test

An evaporation line on a pregnancy test appears after the reaction time. Unfortunately, if you let the test sit for a long period, it’s hard to know whether a faint test line is an evaporation line or a positive result. You’ll have to retake the test if you’re unable to check your results within the recommended time frame.

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It’s also important to note that while an evaporation line appears faint, a faint test line on a pregnancy test doesn’t automatically suggest an evaporation line.

A faint positive test line can also appear if you take a pregnancy test shortly after implantation when your hCG level is low, or if your urine is diluted. This can happen when taking a pregnancy test later in the day after consuming a lot of liquids.

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What’s the best time of day to take a pregnancy test?
Take an at-home pregnancy test as soon as you wake up in the morning when your urine is more concentrated.

Next steps

An at-home pregnancy test can detect a pregnancy, but there’s also the risk of a false negative or a false positive. A false negative can occur if you take a pregnancy test too early, including before a missed period when your hCG levels aren’t high enough.

False positives are less common, but can happen with a chemical pregnancy. This is when an egg implants in the uterus and a miscarriage occurs shortly after.

If you think you’re pregnant, or if you’re confused by the results of an at-home pregnancy test, make an appointment with your doctor to have an in-office test. 

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