The baby has a defect in the abdominal wall to one side of the umbilical cord (usually the right side). Some of the bowel has escaped through this hole and develops outside of the baby's abdomen.

The bowel floats around in the amniotic fluid in the Mother's womb for most of the pregnancy and sometimes the fluid irritates the bowel and makes it inflamed and thick, so that it does not work properly.

Sometimes the bowel becomes twisted as it floats in the fluid and the blood supply is restricted . When the baby is born the bowel is exposed to the air which sometimes causes further inflammation to occur.

The size of the Exomphalos depends on the number of organs exposed. This is usually the bowel but may include the liver and other organs.

The sac containing the exposed organs is usually covered in a protective membrane. The abdominal cavity is sometimes smaller in Exomphalos children due to the organs growing externally. In about 30% of cases Exomphalos occurs in conjunction with other birth defects.