The invention is directed to chimeral fusion proteins having an IgG1 antibody Fc portion and a lysosomal storage enzyme, particularly a Fc-GUS fusion protein useful in treating Sly's disease in an embryo or fetus. The invention is also directed to methods of treating in born errors of metabolism, particularly Sly's disease, in a fetus by delivering to a pregnant mother a Fc-MPS emzyme fusion protein.