As prenatal testing and ultrasound technology have greatly improved, so has our ability to diagnose birth defects and genetic diseases earlier and earlier in pregnancy. Until recently, our only available options were to offer pregnancy termination or wait to see if the baby survived long enough to be treated after birth. But what if we had the capability to intervene before those genetic mutations had a chance to cause their harmful effects, sparing parents from the agony of uncertain pregnancy outcomes and saving children from debilitating diseases? In last year’s “Designer Babies: Hacking Human Embryos” we discussed pre-implantation genetic testing and embryo modification as a means to identify and treat heritable diseases, by correcting harmful gene mutations before a pregnancy even begins. Since then, exciting new research has shown that even after a pregnancy is under way, opportunities still exist for hacking the biological machinery of the fetus to alter its developmental course.This talk will review new and rapidly evolving strategies to treat genetic disease in utero – while the baby is still in the womb - by hijacking the embryologic mechanisms responsible for fetal growth and development. Examples include: - injection of a critical protein into the amniotic fluid surrounding babies with X-linked hypohydrotic ectodermal dysplasia, a genetic condition causing a lack of sweat glands and the life-threatening inability to regulate temperature - transfusion of mesenchymal stem cells into the fetal umbilical cord to treat osteogenesis imperfecta or “brittle bone disease” - in utero blood and bone marrow transplant to treat the fatal hemoglobin disorder alpha-thalessemia major - correcting deformities such as cleft lip and palate by triggering cell signaling pathways ""knocked out"" by genetic mutation