An evaporation line on a pregnancy test refers to a situation in which a pregnancy test may not function exactly as it was designed. When evaporation lines occur on a home pregnancy test, it is possible to read the test as being positive, when you may not actually be pregnant.

To understand what an evaporation line is, it is important to understand some things about home pregnancy tests. Home pregnancy tests are designed to test for the presence of hCG in the woman’s urine stream. Accordingly, most pregnancy tests require the woman to place the test in her urine stream. What can happen is that, after the urine stream crosses the test, the urine on the test may evaporate. This will sometimes leave a mark on the area that gives you the results of the home pregnancy test. Generally, an evaporation line may appear faint, grey, or possibly even as a colorless line. This line can be mistaken for the positive result on the home pregnancy test.

Every brand of home pregnancy test could, in theory, be vulnerable to evaporation lines. For this reason, it is important that you carefully read the instructions that came with your home pregnancy test. Generally, it takes much longer than the indicated time (usually just a few minutes) for evaporation lines to occur. If a home pregnancy test gives you what you believe is a positive result, but it happens after the time that the directions indicate, the result may not be reliable. You should therefore test again after a few days.

Some brands of pregnancy test have gone to providing the user with a symbol, such as a “plus” for pregnant and a “minus” for not pregnant, to help avoid the confusion that can come with evaporation lines. Still, evaporation lines can occur on these sorts of tests, so again it is imperative that you follow the directions of the pregnancy test so that you can properly understand what exactly the results are.

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