This is a glycoprotein hormone, normally produced by a developing placenta implanted in a mother's uterus. It affects the corpus luteum in the ovary, stimulating it to remain active and for it to continue producing progesterone, which is crucial to the maintenance of pregnancy (in the early part of pregnancy before the placenta produces enough progestogens of its own).
HCG consists of two subunits - an alpha and a beta subunit. The alpha subunit is a 92-amino acid sequence that is identical with that of luteinizing hormone (LH), follicle stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). The beta subunit, on the other hand, is unique to HCG and is thus the part that is tested for in pregnancy tests.
HCG also has the immunosuppressive effect of somehow affecting the mother's immune system so that the fetus (by any measure, a foreign object) is not rejected by the mother's body and subjected to immune attack.
HCG is also secreted by certain trophoblastic and germ cell tumours and can be used as a marker to measure the progression of the disease. In a choriocarcinoma, the level of beta HCG is directly proportional to tumour size.