We recently reported the existence of fetal mesenchymal stem cells in first trimester fetal blood. Here we demonstrate that fetal mesenchymal stem cells from as early as eight weeks of gestation can be retrovirally transduced with 99% efficiency without selection. Circulating fetal mesenchymal stem cells are known to readily expand and differentiate into multiple tissue types both in vitro and in vivo, and might be suitable vehicles for prenatal gene delivery. With advances in early fetal blood sampling techniques, we suggest that genetic disorders causing irreversible damage before birth could be treated in utero in the late first/early second trimester by genetically manipulated autologous fetal stem cells.