**8.1. Definition and incidence**

Fetal mediastinal cysts are represented by: pericardial cyst, thymic cyst, esophageal duplication cyst, neurenteric cyst. The incidence of these masses is not known because they are rare pathological entities and only case reports are reported. The pericardial cyst is located at the costophrenic right angle level. The pericardial cyst is covered by mesothelium and has fluid content, and is usually asymptomatic. If the size of the cyst is important, then it can be associated with fetal hydrops, or with the change in heart function at birth [17]. If the pericardial cyst size is reduced in size, it can also regress.

The thymic cyst is very rare, representing 4% of postnatal mediastinal cystic masses [17, 35]. Thymic cysts are asymptomatic, but the prenatal diagnosis is possible.

The esophageal duplication cyst exhibits ectopic gastric mucosa, communicating with the gastrointestinal lumen. Sometimes they may not communicate with the gastrointestinal lumen [17, 36]. The communication with the gastrointestinal lumen is located either above the diaphragm or below the diaphragm.

The neurenteric cyst has a connection with the meninges and the spinal cord and it is usually associated with congenital scoliosis or spina bifida.
