A product of the placenta, and decidua, secreted into the maternal circulation during pregnancy. It has been identified as an IGF binding protein (IGFBP)-4 protease that proteolyzes IGFBP-4 and thus increases IGF bioavailability. It is found also in human fibroblasts, ovarian follicular fluid, and granulosa cells. The enzyme is a heterotetramer of about 500-kDa.