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A pregnancy test detects human pregnancy hormone (human chorionic gonadotropin (hCG)) to determine whether an individual is pregnant.

HCG testing can be performed on a blood sample (typically done in a medical office or hospital) or on urine (which can be performed in an office, hospital or at home.)

The most common tests use markers found in blood and urine, specifically one called human chorionic gonadotropin (hCG.)

Biology
HCG is produced by the syncytiotrophoblast cells of a developing embryo during the biologic event of implantation. The synctytiotrophoblast cells are a precursor to the placenta. HCG produced by the syncytiotrophoblast cells pass into the maternal circulation to maintain the corpus luteum of the ovary. This hormonal balance is necessary for the maintenance and growth of early pregnancy.

In early pregnancy, hCG rapidly increases, doubling every 1.2 to 2 days until the peak in 7-9 weeks of pregnancy. HCG has been detected as soon as 6 days after ovulation in a blood test and is always detectable in the blood by 12 days after ovulation. Urine pregnancy tests vary by manufacturer in how sensitive they are to detecting hCG. Some tests that advertise earliest detection of pregnancy can detect levels as low as 6IU/L while many other commercial tests use cutoffs between 25-50 IU/L.