Home pregnancy tests are among the most popular diagnostic home tests on the market. These tests detect the presence of human chorionic gonadotropin (hCG), a hormone the body makes during pregnancy. If used correctly, these urine pregnancy tests are very sensitive, often claiming to have 99 percent accuracy. However, in some cases these tests provide false-positive or false-negative results. In addition, there are less common reasons -- outside of a healthy pregnancy -- when hCG may appear in the urine.
Pregnancy hCG Levels
For a fertilized egg to stay viable, the embryo needs to travel down the Fallopian tube, into the uterus, and implant in the uterine wall. At this time, hCG levels surge and become detectable in the blood. In fact, blood tests can detect the presence and levels of hCG as early as 8 days after conception. However, it takes a few more days for this hormone to appear in the urine, where the presence, but not the amount, can be measured with a home pregnancy test. In the early weeks of pregnancy, hCG levels increase quickly, by about 50 percent per day, peaking by the 10th week of pregnancy. Since hCG levels increase predictably with pregnancy, and decrease with pregnancy loss, this hormone is commonly checked with hCG blood tests to monitor the health of the pregnancy.
Home Pregnancy Testing
Most home pregnancy tests can accurately detect hCG, and pregnancy, if the urine is tested at least 14 days after conception, or around the time the period is due. Some tests are more sensitive, and can detect a pregnancy a few days earlier, and some are less sensitive, so it's important to read the package directions. A negative test may sometimes provide inaccurate results, however, even if the test is completed and interpreted correctly. For example, testing too soon after conception, when hCG levels are not high enough, or testing urine that is very dilute can lead to false negative test results.
Other Reasons for hCG Production
The hormone hCG can be produced outside of pregnancy. For instance, hCG can be found in the blood and urine of menopausal women, although the levels are not usually high enough to be detected by home pregnancy tests. This hormone is also produced by molar pregnancies, which is a complication of pregnancy in which the baby doesn't develop and abnormal growth occurs, and in ectopic pregnancies, when the fertilized egg implants outside the uterus -- underscoring the importance of seeing a doctor after a positive home pregnancy test. Rarely, the presence of hCG is related to the presence of cancer, particularly those that originate from an egg or sperm.
Most often, the reason to check for the presence of hCG in the urine is to test for pregnancy. If a home urine test is positive for pregnancy, its important to see a doctor to confirm the pregnancy and receive essential prenatal care. In the uncommon situations where hCG is present in the absence of pregnancy, it's important to see a doctor for more specific hCG blood tests and a medical workup. Also see a doctor if a home urine pregnancy test is negative, but signs and symptoms of pregnancy are present.
Reviewed by Kay Peck, MPH RD
- Geburtshilfe and Frauenheilkunde: Strips of Hope: Accuracy of Home Pregnancy Tests and New Developments
- Mosby's Diagnostic and Laboratory Test Reference; Kathleen Pagana, et al.
- U.S. Food and Drug Administration: Pregnancy
- International Journal of Molecular Sciences: hCG: Biological Functions and Clinical Applications