Transposition of the great arteries (TGA) is a birth defect in which the two main arteries carrying blood out of the heart (the main pulmonary artery and the aorta) are switched in position, or 'transposed.'
In babies with TGA, oxygen-poor blood from the body enters the right side of the heart. But, instead of going to the lungs, the blood is pumped directly back out to the rest of the body through the aorta. Oxygen-rich blood from the lungs entering the heart is pumped straight back to the lungs through the main pulmonary artery.
This video explains TGA:
Because a baby with this defect may need surgery or other procedures soon after birth, TGA is considered a critical congenital heart defect.
Other defects
Often, babies with TGA have other heart defects.
This can include a hole between the lower chambers of the heart (a ventricular septal defect) or the upper chambers of the heart (an atrial septal defect) that allow blood to mix so that some oxygen-rich blood can be pumped to the rest of the body.
The patent ductus arteriosus also allows some oxygen-rich blood to be pumped to the rest of the body.