Fetal pleural effusion: In the fetus, excess fluid between the two membranes (the pleurae) that envelop the lungs. The pleural effusion may be unilateral (in one lung) or bilateral (in both lungs) and it may be an isolated finding in an otherwise normal fetus or be associated with generalized edema (hydrops). If untreated, fetal pleural effusion often causes severe, sometimes fatal, respiratory insufficiency in the neonatal period. Treatment is by the prenatal insertion of a thoracoamniotic shunt (to shunt the pleural effusion into the amniotic fluid). Survival after this procedure is over 90% in fetuses with isolated pleural effusion and about 50% in those with associated hydrops.