From Robin Elise Weiss, LCCE,
Your Guide to Pregnancy & Childbirth.
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A.

An evaporation line on a pregnancy test is basically a false positive. While 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.

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 10-30 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.

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