Biliary atresia is a gastrointestinal disorder in which the biliary system is closed or absent. The biliary system is the network of tiny tubular structures and ducts that drain bile from the liver to the small intestine, where it helps the digestive process. Bile is a liquid secreted by liver cells, made up of cholesterol, bile salts and waste products (including bilirubin). Biliary atresia progressively destroys the bile ducts that carry bile from the liver to the intestine, beginning outside the liver and later affecting bile ducts inside the liver. The damaged ducts prevent the draining of bile from the liver; as a result, bile trapped inside the liver causes damage and scarring that can lead to cirrhosis.

As the liver becomes scarred, it presses against the walls of the veins. This constricts the veins and blood cannot pass through them properly. The result is portal hypertension (high blood pressure in the portal vein).

This congenital disorder begins to progress very soon after birth. In its most common form, extrahepatic biliary atresia, ducts outside the liver are affected first.

Although it is relatively rare (occurring in 1 out of every 10,000 live births), biliary atresia is the most common liver disease that requires transplantation. On average, there is one case of biliary atresia out of every 15,000 live births. It occurs slightly more often in females than in males (1.4:1), and affects children of all races. In the United States, approximately 300 new cases of biliary atresia in infants are diagnosed each year.

The cause of biliary atresia is not known. Researchers believe that auto-immune mechanisms may be partly responsible; recent research suggests that biliary atresia could be triggered by a viral infection in susceptible infants. It is also thought that there are two kinds of biliary atresia: Embryonic (fetal) and perinatal. The perinatal type is often associated with a later onset of jaundice, and may be caused by environmental factors.

What is known for certain is that biliary atresia affects only newborns; it is not hereditary; it is not contagious; and it is not preventable. Biliary atresia is not caused by anything the mother did during pregnancy.