Fetal aortic stenosis is an anatomically simple birth defect in which there is partial blockage of the aortic valve, limiting blood flow out of the left side of the heart. If the blockage becomes severe in fetal life, the left heart structures may stop growing and the baby may be born with hypoplastic left heart syndrome (HLHS), a lethal condition without neonatal heart surgery or heart transplantation. Standard treatment for babies born with HLHS includes three major cardiac operations before the age of 6 years. Postnatal outcomes for HLHS are uncertain and vary with patient; however, the few adult survivors are currently suffering substantial medical problems, including a high incidence of neurologic problems. For fetuses with severe obstruction, fetal intervention may improve outcomes by increasing flow through the left heart, thus improving left heart function and encouraging continued growth of the left heart structures. If fetal intervention can preserve left heart function and growth, this should provide a better prognosis for the baby and allow for biventricular repair strategies after birth.