Tetralogy of Fallot presents 4 heart defects which happen together:

A large Ventricular Septal Defect (part of the wall between the front large heart chamber is missing).

Overriding Aorta, which means the door from the left ventricle (the Aortic Valve) originates from both front chambers.

A narrow Pulmonic Valve, because the Aortic Valve originates from both chambers it pushes the Pulmonic Valve aside and makes it narrow.

A large and more muscular Right Ventricle. Since part of the wall between the front chambers of the heart is missing the blue and pink blood get mixed.

Some of the blood with less oxygen instead of going to the lungs to refresh with oxygen bypasses the lungs and goes to the body. This may cause the baby to be blue.

Since the pulmonic valve which allows the blood to go from the right side to the lungs is small, not all the blood can pass to the lungs and some of it is forced to go directly to the body bypassing the lungs.

The severity of this condition depends on how narrow the pulmonic valve is. If it is very narrow the baby would be very blue from birth and may need immediate surgical attention.

If it is mild the child may grow to one to two years old without needing anything. Most babies fall between these two varieties and turn blue with mild activity such as feeding or crying.