Physicians do not yet understand what causes the congenital heart defects that are frequently underlying the baby’s bluish tint (cyanosis). Studies have shown that some congenital heart defects tend to run in families. In addition, a problem pregnancy (e.g., diabetes) or use of certain medications during pregnancy may alert the physician to a possible congenital defect. Some risk factors may prompt the obstetrician to suggest a fetal echocardiogram during the pregnancy to examine the infant's heart in-utero.

In terms of blue baby, there are five cardiac-related congenital diseases, known as the five Ts,that cause most cases of blue baby. They are:

• Tetralogy of Fallot. A condition that involves the “tetralogy” of the following four elements:
  
  
• Ventricular septal defect (VSD). A hole in the wall (septum) between the heart’s two lower chambers (ventricles). The VSD creates a pathway between the right and left ventricle. This results in blood flow between the two chambers mixing together prior to going out to the lungs and body. 
  
  
  
• Pulmonary stenosis. A narrowing stenosis (narrowing) of the pulmonic valve and the muscular region below the valve exists. This decreases the amount of oxygen–poor blood from the right ventricle that can squeeze through the narrowed opening and travel through the pulmonary artery to the lungs. Thus, there is a decreased blood flow to the lungs and a corresponding increase in the amount of blood shunted to the left side of the heart.
  
  
• Hypertrophy (thickening) of the right ventricle. A thickening of the muscle tissue of the right ventricle below the pulmonary valve due to pulmonary obstruction (as described above).
  
  
• Displaced or deviated aorta. Much of the oxygen–poor blood in the right ventricle cannot squeeze through the pulmonary valve and travel to the lungs as it should. This forces the blood to travel up through the displaced aorta, which opens directly into both the right and left ventricles. Therefore, the aorta is receiving both oxygen–poor blood from the right ventricle and oxygen–rich blood from the left ventricle, and carries this mixture to the rest of the body. Because the body is receiving so much oxygen–poor blood, the skin of the child often has a bluish tint (cyanosis).
  
  
• Transposition of the great arteries (TGA). A congenital heart defect (present from birth) in which the “great arteries,” the aorta and the pulmonary artery, are switched, or transposed. In this defect, the aorta leaves from the right ventricle (carrying oxygen–poor blood to the body) and the pulmonary artery leaves from the left ventricle (carrying oxygen–rich blood right back to the lungs). The baby may develop cyanosis because oxygen–poor blood, full of waste products from the cells and organs, is pumped directly back into the body before oxygenation in the lungs can take place.

• Total anomalous pulmonary venous return (TAPVR). A condition in which the pulmonary veins that bring oxygen-rich blood back from the lungs to the heart are not connected to the left atrium. Instead, these veins drain oxygenated blood into the right atrium where it mixes with oxygen-poor blood coming back from the body. This mixture of blood must pass through a hole in the septum between the right and left atria to reach the left ventricle (which can then pump the blood to the rest of the body).
  
  
  
  
• Truncus arteriosus. A condition in which a single great vessel arises from the heart. The aorta, pulmonary arteries and coronary arteries are all part of this common vessel, which allows the complete mixture of oxygen-rich and oxygen-poor blood. This defect is always accompanied by a ventricular septal defect, another heart defect.
  
  
• Tricuspid atresia. A condition in which the tricuspid valve never developed. This condition impedes or totally obstructs blood flow between the right atrium and the right ventricle. As a result, all the oxygen-poor blood mixes with oxygen-rich blood through an atrial septal defect. This mixture then circulates to the body.

The last category is sometimes referred to as the sixth “T”, which stands for “tons of other diseases” that might be the underlying cause of blue baby. These might include:

• Hypoplastic left heart syndrome (HLHS). A condition in which the left side of the heart (including the aorta, aortic valve, left ventricle and mitral valve) is not totally developed. As a result, blood flow, pumping and most other cardiac functions are severely compromised. This can lead to cyanosis because the left ventricle is not adequately pumping enough oxygen–rich blood to the body, and all of the blood is a mixture of oxygen-rich and oxygen-poor blood. 
  
  
• Critical pulmonary stenosis (severe narrowing of the pulmonary valve) or pulmonary atresia (complete closure). Normally, this valve allows blood to flow from the right ventricle to the lungs. When severely narrowed or closed, less blood flow reaches the lungs, leaving less oxygen–rich blood available to circulate through the body.
  
  
• Coarctation of the aorta. A condition in which the aorta is narrowed, usually in a specific place shortly after it emerges from the heart. If the narrowed portion is able to restrict blood flow to the baby, then cyanosis may result.