Hydrothorax is the accumulation of fluid in the pleural space. A large fetal pleural effusion (hydrothorax) was identified within the right hemithorax in a 22-year-old woman at 39 weeks of gestation. Because the fetal hydrothorax was detected at 39 weeks of gestation, we did not perform any intervention antenatally. A male fetus was delivered by an elective cesarean section at 39 weeks of gestation. He did not require intubation, ventilation or resuscitation in the operating room, and had a good health status. The baby was followed-up in the neonatal intensive care unit. Six days after the birth, the pleural effusion (hydrothorax) was completely resolved spontaneously. Conservative management may be regarded as an option for isolated fetal hydrothorax in uncomplicated, stable cases without hydrops.