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What Is an Evaporation Line on a Pregnancy Test?

author image Sharon Perkins
A registered nurse with more than 25 years of experience in oncology, labor/delivery, neonatal intensive care, infertility and ophthalmology, Sharon Perkins has also coauthored and edited numerous health books for the Wiley "Dummies" series. Perkins also has extensive experience working in home health with medically fragile pediatric patients.
What Is an Evaporation Line on a Pregnancy Test?
A woman holds up a home pregnancy test. Photo Credit: AVAVA/iStock/Getty Images

Urine pregnancy tests have made it possible to find out if you're pregnant in the privacy of your own bathroom. But as easy as they are to use, the results can sometimes be confusing, especially if you wait longer than the recommended time to read the results or pick it out of the trash for a second look. An evaporation line, which appears when urine dries, might look like a positive test, but it is not.

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Understanding the Lines

Urine pregnancy kits test human chorionic gonadotropin, or hCG, which is produced by the placenta as it grows. Most home pregnancy tests can detect hCG levels that are 25 mIU/ml and higher. Pregnancy tests have two lines: a control line that will darken when you wet it, regardless of whether you're pregnant, and a second line that tests for hCG. If the second line darkens to match the control line, the hCG in the urine has reacted with the hCG antibodies in the test strip and you're pregnant. If it doesn't darken within the allotted time, generally around five minutes, you're not pregnant.

Evaporation Lines

When the test line begins to dry, a faint, colorless or gray line known as an evaporation line might appear. Since it takes longer than a few minutes for the test to dry, you might not notice this for 10 minutes or longer, according to Craig Medical. If you think you might be pregnant, do another test in 48 hours as soon as you get up or see your doctor for a blood test.

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