Gonadotropin (also designated choriogonadotropin) is a hormone produced by the placenta in the first trimester of pregnancy and exists as a heterodimer formed from a common (alpha) chain and a unique (beta) chain. The unique (beta) chain confers biological specificity to thyrotropin, Lutropin, follitropin and gonadotropin. The secreted (alpha) subunit maps to human chromosome 6 and the (beta) subunit maps to human chromosome 19. Gonadotropin stimulates the ovaries to produce and maintain normal levels of the steroids essential for maintaining pregnancy, including estrogen and progesterone. Gonadotropin is a member of the cystine knot growth-factor superfamily, a group of proteins that contain a distinct arrangement of six cysteine residues and are expressed in placenta. The proper secretion and dimerization of gonadotropin depends on the conformation of the cystine knot, although biological activity is independent of this conformation.