Rapid spontaneous mid-trimester resolution of marked isolated bilateral fetal pleural effusions of undetermined etiology

Fetal pleural effusion is considered a rare congenital anomaly with a reported incidence of between 1:10,000 and 1:15,000 deliveries. Fetal pleural effusion may be unilateral or bilateral, primary or secondary, congenital or acquired, isolated or a component of fetal hydrops. The clinical course of fetuses with pleural effusion is unpredictable with outcomes ranging between spontaneous resolution (in between 9 and 22% of cases), iatrogenic or spontaneous prematurity, and stillbirth. Fetal pleural effusion (fetal hydrothorax) can rapidly progress to severe fetal hydrops, a complication associated with a mortality rate of approximately 50%. We present an unusual case of isolated marked bilateral pleural effusions of undetermined etiology. During preparation for possible fetal thoracocentesis for drainage in the event of the onset of fetal hydrops, rapid complete spontaneous resolution occurred within 5 days. Continued expectant management was performed with weekly sonographic surveillance and twice weekly fetal testing, with subsequent normal neonatal outcome. This case describes rapid worsening of fetal pleural effusions with subsequent spontaneous resolution, and emphasizes continued expectant management with close fetal surveillance.