Not all home pregnancy tests have the same level of sensitivity to hCG (human chorionic gonadotropin), the pregnancy hormone, so some will show a positive result early in a woman’s pregnancy, and some won’t register positive until up to a week after a missed period. What the majority of tests do have in common is that positive results doesn’t always last long enough to show off to friends and relatives. That doesn’t mean you’re not pregnant, just that the type of test you took is subject to a time limit for reading the results.
A lot has changed with home pregnancy tests since they were first made available to consumers in the late 1970s. Today’s pregnancy tests are less expensive, more reliable, easier to use and can detect the pregnancy hormone, hCG at very low levels. This means women can, theoretically, have a positive result on a home pregnancy test as early as the first day of a missed period.
Mechanics of an HPT
Most home pregnancy tests have two windows: a control window and a test window. The test strip is lined with paper treated by chemicals that are reactive to certain substances. A portion of the control window is reactive to any type of liquid (in this case, urine) and a colored line will appear when urine wets the paper. The test window, however, is reactive to hCG and will only show a line, a “+” or the word “pregnant” if the urine contains hCG.
Time Lapses and False Positives
The chemical reaction, however, isn’t always permanent and will fade, usually within an hour or two of taking a test. In other cases, a negative test will look positive after an hour or so; where at the initial read there was only a line in the control window, a few hours later a line appears up in the test window, too.
hCG and Evaporation Lines
It’s not hCG causing the line in the test window, though, it’s an evaporation effect. The evaporation line will appear on the test line as urine dries, simply because that area of the strip has a different density then the rest of the paper in the window. For this reason, the results of a typical home pregnancy test are usually only accurate for the first hour after the test is taken.
Digital Pregnancy Tests
Digital pregnancy tests are an exception to this rule. While some digital pregnancy tests still provide a woman with a test strip with the typical “+” or “-” readout she can eject from the device and save, many also show the words “pregnant” or “not pregnant” on the screen to dismiss uncertainty. They are also easier to time–while waiting for results the screen displays a blinking hourglass. When the test is ready, the hourglass is replaced by the words (or the word “error” if the test wasn’t performed correctly). The results of these digital tests will remain for as long as you keep the test in hand, so if you’re looking for a souvenir for the baby book, retake the test using a digital HPT kit.