Choriogonadotropin is a hormone produced by the placenta in the first trimester of pregnancy and exists as a heterodimer formed from a unique β chain and an α chain common to all gonadotropins. The unique β chain confers biological specificity to choriogonadotropin, luteinizing hormone and follicle stimulating hormone. The secreted α subunit maps to human chromosome 6 and the β subunit of choriogonadotropin maps to human chromosome 19. Choriogonadotropin stimulates the ovaries to produce and maintain normal levels of the steroids essential for maintaining pregnancy, including estrogen and progesterone. Choriogonadotropin 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 choriogonadotropin depends on the conformation of the cystine knot, although biological activity is independent of this conformation.