Perhaps it has happened to you before - you and your husband were huddled in the bathroom, one staring at the home pregnancy test and the other reading the instructions. Unsure of your results, you debated over just how pink the line has to be in order for it to indicate a positive pregnancy. Like many, I too pondered the plausibility of negative results. I have heard plenty of whispers of women taking three or four tests to “verify” results, or taking tests that proclaim them not pregnant, only to find out weeks later the tests were wrong. How often does this really happen? Is there really that large a margin of error in commercial home pregnancy tests? Is it more likely to get a false negative or a false positive? How sure can you really be when looking at that little plastic stick?

There are dozens of home pregnancy tests on the market. Whether they show their results in lines, symbols, or words, by activating dye or digitally, they essentially work in the same way. These tests are designed to detect levels of human chorionic gonadotropin (hCG) in the urine of a pregnant woman. These tests proclaim greater than 99% accuracy, and state they can detect these levels on the first day an expected period is missed, if not earlier. While this sounds fool-proof, the reality is these tests are not as reliable as women would like to think. Most women who take home pregnancy tests will receive inaccurate results at least once, generally due to improper usage. It is essential women take the test with their first morning urine, which is the most concentrated and will, therefore, contain the most hCG. Some women will take home pregnancy tests and see only a light positive response or a negative response only to find out later they are, indeed, pregnant. The variation in effectiveness is due to the fat that most home pregnancy tests are not calibrated to detect low levels of H-hCG, a variant of hCG which is more indicative of early pregnancy.

False-negative results are quite common, as many women will take the test too early, not use first morning urine, or not wait long enough to evaluate the results. False-positive results are essentially unheard of as that would be an indication of a hormone being present that should not be present if the woman is not pregnant. Even women who see very light positive results or still suspect pregnancy after receiving a negative result, should contact their physician.

Source: Butler, Stephen et al. Detection of Early Pregnancy Forms of Human Chorionic Gonadotropin by Home Pregnancy Test Devices, Clinical Chemistry, December 2001, Volume 47, Issue 12, pp 2131-2136.