Maternal Hyperoxygenation in Fetal Left Heart Hypoplasia

Inclusion Criteria: * Group A, C: Fetuses with hypoplastic left sided structures or at risk of coarctation of the aorta and ALL of the following * Any mitral annulus, aortic annulus, left ventricular end-diastolic dimension, or aortic diameter z-score less than or equal to 3.0 * Right-left ventricular size discrepancy with no other explanation of discrepancy * Retrograde blood flow in the aortic arch from the ductus arteriosus * Left to right flow across the foramen ovale * Group B: Healthy Fetal controls * Mothers undergoing screening fetal echo for family history of CHD with a normal echo. * Mothers undergoing fetal echocardiography for suspected heart disease with a normal echo * Mothers evaluated in the fetal center with normal ultrasound * Group D: Fetuses with hypoplastic left sided structures or at risk of coarctation of the aorta and any of the following * Any mitral annulus, aortic annulus, left ventricular end-diastolic dimension, or aortic diameter z-score less than or equal to 2.0 * Right-left ventricular size discrepancy with no other explanation of discrepancy * Continuous Doppler flow in the aortic arch concerning for coarctation * Significantly less aortic flow than pulmonary artery flow * Severe atrial septal aneurysm with possible obstruction of mitral valve flow * Left superior vena cava to coronary sinus with dilated coronary sinus. * Retrograde blood flow in the aortic arch from the ductus arteriosus * Hypoplastic left heart syndrome (HLHS) and variants of HLHS * Total anomalous pulmonary venous return Exclusion criteria: * Multiple gestations * Persistent arrhythmia * Very poor ultrasound images, defined by the inability to reliably measure/evaluate all included cardiac structures (valve annuli, branch pulmonary arteries, PFO, and arch) * Major extra cardiac anomalies * Aneuploidy * Maternal conditions that may alter fetal hemodynamics, including moderate to severe HTN requiring medication in pregnancy, preeclampsia, major or unrepaired maternal congenital heart disease, obstructive sleep apnea, severe asthma (non-responsive to inhaled steroid therapy), restrictive lung disease, severe anemia, maternal chronic renal disease known placentation abnormality (complete placenta previa, accrete, or percreta), and antiphospholipid ab syndrome