Trial of Maternal Oxygen for Fetal Left Heart Hypoplasia

The purpose of this study is to determine if maternal hyperoxygenation is an effective treatment for fetal left heart hypoplasia versus room air (placebo). This will be determined by measuring how well a baby's heart valves and their surrounding tissue are growing and functioning. In addition the investigators will examine brain growth using fetal ultrasound and MRI, and MRI of the child's brain after they are born to determine if there is greater neonatal brain maturity or mothers receiving oxygen compared to fetuses of mothers not receiving oxygen. Of note, the trial was initially randomized. However, due to low sample size and hesitation about randomization, the trial was converted to an open label study, allowing families opting for oxygen therapy to be in the intervention arm.