Why is transposition of the great vessels such a big concern for a fetus?

Blue newborn! Tga is a very severe cardiovascular anomaly because it can cause very low oxygen concentration in the blood going to the coronaries and carotids and prove lethal if untreated promptly and expertly. If a ventricular septal defect is present, the newborn can be more stable and interventions can be more planned and less emergent. Fetal life is ok - trouble starts as soon as the cord is clamped.
Poor oxygen delivery. At birth the lungs are required for delivering oxygen to the body. The blood normally flows from the right side of the heart to the lungs to the left side of the heart and out to the body. With transposition, blood flows to the body from the right side of the heart without picking up oxygen in the lungs first. Oxygenated blood in the left side goes back to the lungs.