As the number of inborn errors of metabolism that can be diagnosed prenatally by direct DNA analysis increases, so will the number of patients interested in having prenatal diagnosis for these conditions. The phenotypic expression of these inherited inborn errors of metabolism in theory could be prevented by the in utero transplantation of donor stem cells. It is the authors' opinion that ultrasound-guided celocentesis performed in the timed-pregnant baboon animal model could have an important role for determining if stem cell transplantation in the nonfunctional immunocompetent embryo has any clinical potential.