Why do in utero stem cell transplants sometimes fail?

In utero stem cell transplantation promises a novel therapeutic approach to those genetic disorders that can be diagnosed in early pregnancy and that could lead to either severe disability or death, available scientific evide nce suggests that such procedures could achieve clinically relevant levels of engraftment with donor cells and that the resulting sustained chimerism is potentially long lived. However, the relatively few cases performed so f ar have not borne out initial hopes, and both the source and the type of ce ll preparation to be transplanted and the choice of disorders to target wit h this therapy remain controversial.