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Pregnancy (hCG)

Pregnancy testing and hCG

Evidence for testing to detect pregnancy has been found as far back as ancient Egyptian cultures. In the early twentieth century human chorionic gonadotropin (hCG) was identified as the major hormone associated with pregnancy.  The first pregnancy test based on hCG was called the ‘Rabbit test’ and were based on a bioassay in which urine was injected into young female rabbits. If hCG was present the rabbit ovaries would be enlarged.  Immunoassays for hCG were developed around 1960 and were latex agglutination assays. More recently enzyme-linked immunoassays were developed. The most commonly used pregnancy tests either in doctor's offices or over-the-counter (OTC) tests are based on lateral flow tests.

Human chorionic gonadotropin (hCG)

hCG is a small glycoprotein hormone produced by cells that form the placenta. It stimulates progesterone production by the ovary and the placenta and helps maintain the pregnancy. hCG is composed of two different subunits, alpha (α) and beta (β). The alpha subunit is common to all members of the glycoprotein-hormone family and the beta subunit is unique to hCG and confers its biological activity. All pregnancy tests are based on the beta subunit. There are many different forms of hCG based on the degree of glycosylation and different proteolytic forms. Not all hCG tests detect or measure all these forms of hCG equally. Although measurement o hCG is primarily done to detect pregnancy, hCG is used for other purposes such as detecting certain types of cancer.

Pregnancy testing

Pregnancy tests can either be urine or blood tests, but the most commonly used are urine tests. Urine pregnancy tests are accurate and they carry almost the same accuracy as blood pregnancy tests but they are considerably more economical. Urine tests can be taken in a doctor’s office or in the convenience of the home. Serum tests are usually done to quantitate hCG. This can be used to determine the gestational age. Pregnancy tests can detect a pregnancy as early as 10 days after conception, especially if a first morning urine sample is used.  False-positive tests can occur if for example the test is read improperly or if a woman is receiving hCG injections to treat infertility. False negative results can occur if the test is used too early in the pregnancy. A false negative test can also occur late in pregnancy due to very high hCG levels that can cause interference in the test.

Recommended Reading

American Pregnancy Association: http://americanpregnancy.org/pregnancycomplications/groupbstrepinfection.html

http://americanpregnancy.org/duringpregnancy/hcglevels.html

CDC website:

http://www.cdc.gov/pregnancy/