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The Accuracy of Urinalysis for Determining Pregnancy

written by: Dr Mike C • edited by: Emma Lloyd • updated: 5/22/2010

Urinalysis offers a quick and easy method to test for pregnancy. The accuracy of urinalysis for pregnancy determination will depend on the length of time after the embryo has implanted in the uterus as implantation triggers a rise in the HCG (the "pregnancy hormone") detected by the test.

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    Hormonal Changes that Occur during Pregnancy

    When a sexually active woman who is used to having regular menstrual periods finds that her period is late, the natural question to ask is: could I be pregnant? For many women, the news that they are expecting a child would be joyful; however, for some women the news might be unwelcome for a variety of reasons.

    Irrespective of whether a pregnancy is planned or not and particularly if the women feels unable to carry it to term, it is important to get early confirmation that she is, indeed pregnant.

    Home pregnancy testing kits are widely available in the western world and permit a women to find out if she is pregnant or not easily and discretely. These tests are based on the analysis of a urine sample by an enzyme-linked immunosorbent assay (ELISA). The accuracy of urinalysis for pregnancy determination is usually very high and therefore the tests are reliable as well as convenient.

    When a woman conceives a baby, her body starts to produce a hormone called human chorionic gonadotropin (HCG) once the fertilised egg has implanted itself in the wall of the uterus. This stage will typically occur six days after conception has taken place, but in up to 10% of women this will happen later.

    The levels of HCG steadily build up during pregnancy, rising to a peak after 8 to 11 weeks before falling to a stable level which is maintained throughout the remainder of the gestation. HCG is made by the cells that form the placenta which nourishes the egg during the early stages of pregnancy.

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    The Accuracy of Urinalysis for Determining Pregnancy

    Urinalysis is the process of screening a sample of urine for the presence of certain substances, such as HCG. In a home pregnancy testing kit, the test strip is typically held in a plastic cassette which is either held in the stream of urine as the woman urinates or urine is collected and then a few drops are transferred to the test using a plastic dropper (included in the kit). In either case, the result can be read after just a few minutes.

    In older tests, a coloured band will show in the test window if the test is positive, a second window, the control window should always show a bar if the test is working properly. So-called “digital” pregnancy tests work on the same basis, but the result will cause the word “Pregnant” or “Not Pregnant” to be displayed on a small screen, making interpretation easier. From what we have discussed earlier, it is obvious that the accuracy of urinalysis for pregnancy determination will depend on the concentration of HGC present in the urine.

    With traditional tests, the presence of the confirmatory line could be open to debate if the test was done shortly after the embryo implanted in the uterus. For this reason, doctors will usually confirm an early stage pregnancy via a blood test assay for HGC which is more sensitive than the urinalysis test. This can be particularly important, for example, in IVF pregnancies where certain drugs may need to be administered in the early stages of the pregnancy to support it and enhance the chances of the embryo developing to full-term.

    For the best and most accurate results, a blood test needs to be performed by a health care professional and the blood sample will usually need to be analysed in a laboratory before the result is available.

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    References

    1. US Department of Health and Human Services: http://www.womenshealth.gov/faq/pregnancy-tests.cfm#a
    2. American Pregnancy Association: http://www.americanpregnancy.org/duringpregnancy/hcglevels.html