Pregnancy Test Evaporation Line

Woman holding home pregnancy test and looking worried

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When taking a pregnancy test, you really want to make sure that you are following the instructions and doing it correctly. One of the most common mistakes on a pregnancy test is misreading it, and an evaporation line is one way to misread a pregnancy test.

The evaporation line on a pregnancy test comes when the urine used in the sample evaporates after a certain amount of time. This is normal and a part of the process but leaves behind a colorless but distinct line. The evaporation line does not show up until after the pregnancy test reading window of time to accurately get your results. So you will not need to worry about an evaporation line if you read the pregnancy test within the time frame specified in the instructions that came with your test. For most pregnancy tests, the results will first appear within five minutes.

Avoiding an Evaporation Line 

An evaporation line on a pregnancy test is basically a false positive. While true false positives and false negatives are fairly rare, the evaporation line is the closest most women will come to having an error on their pregnancy tests. You can completely avoid this error by reading the instructions and using a watch when doing your pregnancy test.

The evaporation line is basically caused by reading a pregnancy test after the time limit advised by the manufacturer has passed. This time limit is usually clearly stated in the home pregnancy test kit instructions. The majority of the pregnancy test kits advise you to read the results within a maximum of 10 minutes after the urine test results. The evaporation line is only found on urine pregnancy tests. Using a digital pregnancy test will remove the risk of misreading a pregnancy test because of the way the test results announce if you are pregnant or not. This is also a great reason not to pull out a pregnancy test from the day before.

You will not have an issue with an evaporation line if you use a digital pregnancy test. This is one of many reasons mothers choose to use a digital pregnancy test. They can avoid the worry about misreading the tests in a positive or negative manner.

What Else Could Cause a Light Line?

A light line can be caused by a few things besides an evaporation line. This can include a diluted urine sample, meaning somehow the concentration of hCG wasn't high enough to give a full-blown positive. You could also get a very light line if you take the test too early in the pregnancy, since your hCG levels may not have risen very high yet. Though it's important to note that a positive pregnancy test should be considered positive.

5 Sources
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By Robin Elise Weiss, PhD, MPH
Robin Elise Weiss, PhD, MPH is a professor, author, childbirth and postpartum educator, certified doula, and lactation counselor.