Mary Calvagna, MS
Whether it is a blood or urine test, all pregnancy tests work the same way—by testing for the presence of human chorionic gonadotropin (hCG). hCG is a hormone produced in the placenta after egg fertilization. Its concentration doubles every two or three days, peaking around the eighth week of pregnancy. In-home pregnancy kits test urine for hCG.
Many tests can start to detect pregnancies as early as 10-15 days after conception or one day after a missed menstrual period. But since the level of hCG continues to rise, the test becomes more sensitive over time. So on the first day after a missed period, the urine pregnancy test may only detect some pregnancies, but one week later it would likely detect most pregnancies. If you tested and the result was negative, and you still have not started your period, wait a few days and test again.
It is important that you read the directions. Not all in-home pregnancy kits are used in the same way. With some kits, you will urinate in a cup and then use a dropper to place a small sample of your urine on the test area. Other kits require you to urinate directly on the test stick. The results usually take about five minutes to display.
In-home pregnancy tests are very accurate if used appropriately 1 week after your missed period. However, if used incorrectly, if the instructions are not followed, or if you use the test too early, then the results are much less accurate.
It is important that you follow the directions and understand how to interpret the results. Manufacturers state that in-home pregnancy tests can be used as soon as one day after a missed period, but understand that the tests are much more accurate if you wait a week after a missed period.
When an error does occur, more often than not, it is a false-negative—meaning the test says you are not pregnant when you are. If the test result is negative but you are experiencing early signs of pregnancy, it is best to see your healthcare provider. The earlier that you begin prenatal care, the better it is for both you and your baby.
If your home pregnancy test is positive for a pregnancy, it is very likely to be correct. It is extremely rare that a test would give you a positive result if you were not really pregnant.
The most common cause of a missed period is pregnancy. But what if you have missed a period and are not pregnant? Some other causes of missed periods include but are not limited to:
American Congress of Obstetricians and Gynecologists
Women's Health—US Department of Health and Human Services
The Canadian Women's Health Network
The Society of Obstetricians and Gynaecologists of Canada
Pregnancy testing. EBSCO DynaMed website. Available at: http://www.ebscohost.com/dynamed. Updated January 3, 2013. Accessed March 4, 2014.
Pregnancy tests fact sheet. Office on Women's Health website. Available at: http://womenshealth.gov/publications/our-publications/fact-sheet/pregnancy-tests.html. Updated July 16, 2012. Accessed March 4, 2014.
Taking a pregnancy test. American Pregnancy Association website. Available at: http://americanpregnancy.org/gettingpregnant/takingapregnancytest.html. Updated March 2011. Accessed March 4, 2014.
Last reviewed March 2014 by Michael Woods, MD
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