A heterodimeric glycoprotein hormone of 57 kDa consisting of a noncovalent bound a (92 amino acid residues) and a distinctive β (134 amino acid residues) subunits found in the blood of pregnant females. The major function of this hormone is to stimulate the production of progesterone by the corpus luteum. This ensures a continuous supply of ovarian progesterone until the placenta has developed to the point when it can generate its own supply of progesterone.