The wide, flat muscle that separates the chest and abdominal cavities is called the diaphragm. The diaphragm forms when a fetus is at 8 weeks’ gestation. When it does not form completely, a defect, called a congenital diaphragmatic hernia (CDH), is created. This is a hole in the muscle between the chest and the abdomen.
The majority of CDHs occur on the left side. The hole allows the contents of the abdomen (stomach, intestine, liver, spleen, and kidneys) to go up into the fetal chest. The herniation of these abdominal organs into the chest occupies that space and prevents the lungs from growing to normal size. The growth of both lungs can be affected. This is called pulmonary hypoplasia.
While in the uterus, a fetus does not need its lungs to breathe, because the placenta performs this function. However, if the lungs are too small after the baby is born, the baby will not be able to provide itself with enough oxygen to survive.
The depiction above is of a SEVERE CDH.