Human chorionic gonadotropin (hCG) is a hormone produced during pregnancy. It supports pregnancy by promoting the production of progesterone, which helps prepare the uterine lining for implantation. hCG is made up of cells that eventually form the placenta, which provides nutrition to the fertilized egg and connects it to the uterine wall.
hCG is a dimer composed of a unique 145 amino acid beta-subunit and a 92 amino acid alpha-subunit. The alpha-subunit is identical to those found in luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). The genes for the alpha and beta-subunits are located on different chromosomes (chromosomes 6 and 19, respectively). After synthesis, the alpha and beta-subunits bond noncovalently before being released into the bloodstream.

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In males and nonpregnant females, normal hCG levels are less than 2 IU/L. For pregnancy detection in females, hCG levels indicate the following:

• Negative: 5 IU/L
• Indeterminate: 5-25 IU/L
• Positive: 25 IU/L