Fetal Intervention for Aortic Stenosis and Evolving Hypoplastic Left Heart Syndrome

Inclusion Criteria: * Pregnant woman carrying a fetus with normal cardiac segmental anatomy and severe aortic stenosis as defined by: * Dysplastic/stenotic aortic valve with forward flow, with or without gradient, and no significant insufficiency * If mitral insufficiency (incomplete closure of the mitral valve)is present, left ventricle systolic pressure calculation must be ≥ a normal systemic blood pressure (BP) for gestational age * Retrograde aortic arch flow * Left to right atrial shunting * Left ventricle length no less than 90% the length of the right ventricle * Maternal age ≥ 16 years of age; * Gestational age: 17 0/7-30 6/7 weeks' gestation as determined by clinical information and evaluation of first ultrasound. (Aortic valve dilation procedure cannot be performed until 18 0/7 weeks); * Normal karyotype with written confirmation of results. Results by fluorescence in situ hybridization (FISH) for aneuploidy will be acceptable if the patient is at 24 weeks or more; non-invasive testing is acceptable (maternal serum testing for cell-free fetal DNA, currently commercially available). * Singleton pregnancy; * Able to travel to study site for study evaluation, procedures, and visits; * Support person to travel and stay with patient (support person will be required to sign the support person consent form); * Has received pre-authorization for insurance for fetal intervention or has the ability to self-pay for study treatment Exclusion Criteria: * Failure to meet all inclusion criteria; * Multi-fetal pregnancy; * Insulin dependent pregestational diabetes; * Fetal anomaly not related to aortic stenosis. A detailed fetal anatomic ultrasound will be conducted before consideration for the study and if the finding is abnormal, the patient will be excluded; * Current or planned cerclage or documented history of incompetent cervix; * Placenta previa or placental abruption; * Short cervix (\< 20mm) measured by cervical ultrasound; * Previous spontaneous delivery prior to 37 weeks of a singleton pregnancy. If the study candidate had intact membranes and was induced, this is not considered spontaneous. Stillbirths prior to 37 weeks are not exclusionary. * Obesity as defined by body mass index of 35 or greater; * Maternal-fetal Rh isoimmunization, Kell sensitization or a history of neonatal alloimmune thrombocytopenia; * Positive maternal HIV status. This is due to the increased risk of transmission to the fetus during the maternal-fetal procedure. If the patient's HIV status is unknown, the patient must be tested and found to have negative results before she can be entered into the fetal treatment group; * Known Hepatitis C positivity. If the patient's Hepatitis C status is unknown, she does not need to be screened; * Other maternal medical condition which is a contraindication to surgery or general anesthesia, such as asthma, cardiac disease, the refusal of a blood transfusion, or a previous hysterotomy in the active segment of the uterus; * Patient does not have a support person (e.g., husband, partner, mother), or that support person is unwilling to sign the support person consent form; * Inability to comply with the travel and follow-up requirements of the study; * Patient does not meet other psychosocial criteria to handle the implications of the study; * Participation in another intervention study that influences maternal and fetal morbidity and mortality; * Maternal hypertension which would increase the risk of preeclampsia or preterm delivery (including, but not limited to: uncontrolled hypertension, chronic hypertension with end organ damage and new onset hypertension in current pregnancy).