Home pregnancy tests measure the amount of a hormone called human chorionic gonadotrophin or hCG in the urine. Women begin producing this hormone from the beginning of conception and reach a peak concentration at eight to ten weeks. All tests use a stick with a specially treated test strip that detects hCG when it comes in contact with a woman's urine. Within minutes, women can read the results in the test window of the stick.
Traditional
Traditional home pregnancy tests will show the results as either pink or blue lines to indicate a positive pregnancy. These are all based on the original e.p.t. home pregnancy test; the first test approved by the U.S. Food and Drug Administration for home use in 1976.
After the test strip comes in contact with urine and the allotted time has gone by the woman reads the results in the test window. For example, Accu-clear is a standard stick test that shows a positive result as a pink line. Clearblue Easy +/- shows the results as either a pink plus, for positive pregnancy, or a pink minus sign for negative. First Response Rapid Result uses a pink line for a positive pregnancy test. There are a number of other brands that use the traditional line for a positive result.
Digital
Digital home pregnancy tests are newer in the timeline of at-home tests. Instead of a pink or blue line, a woman reads the result as a word. Studies in the 2008, "Current Medical Research and Opinion" reported that digital tests prove more accurate than traditional tests, due to the ease in reading the results.
First Response Gold Digital is one of these tests that will display the results in the test window as either "yes" or "no" to pregnancy. Clearblue Easy Digital and e.p.t. Certainty are also home pregnancy tests of this type. The results display either the words, "pregnant" or "not pregnant".
Early
Early home pregnancy tests prove the most sensitive at detecting the presence of hCG in a woman's urine. Because they can detect the hormone at much lower concentrations, women can use them earlier, when their bodies have just begun to produce hCG.
First Response Early Result is one of these tests. Studies in the American Journal of Obstetrics and Gynecology found that it can detect concentrations of hCG at 6.5 mIU/ml. The manufacturer states that this test can be used six days before a woman's missed period.
Other home tests considered "early" detect hCG concentrations of 25 mIU/ml.
Variations in hCG
All pregnant women produce hCG at different rates and levels, therefore home pregnancy tests are highly dependent on the concentration of hCG in the woman's urine. Each day after conception the amount of hCG increases, making home pregnancy tests more effective at detecting a pregnancy as a woman nears her period due date.
References
- Office of NIH History: The History of the Pregnancy Test
- "Current Medical Research and Opinion": Comparison of accuracy and certainty of results of six home pregnancy tests available over-the-counter
- "American Journal of Obstetrics and Gynecology": Accuracy of home pregnancy tests at the time of missed menses
- "Parents": 10 Home Pregnancy Tests
