Article in this week's Science Magazine: Researchers are designing the first clinical trials of stem cell treatments for fetuses afflicted with rare bone and blood diseases. The trials, still being planned, involve arguably the trickiest patient population there is: pregnant women and their fetuses. And although researchers once thought that the fetal immune system would readily accept foreign cells, they now know that it is not that simple. But some fetuses have already been treated on an ad hoc basis, with encouraging results. After decades of hopes raised and dashed, pediatricians, immunologists, and others are cautiously hopeful that new biological insights and a push for treatment from parents-to-be could turn the tide for prenatal stem cell therapy.