A 23-year-old woman in her third pregnancy was referred at 28 + 2 weeks because of fetal hydrops and a suspected placental tumor. Ultrasound examination confirmed severe polyhydramnios, placentomegaly and a large chorioangioma of 104 × 84 × 88 mm. The fetus had hydrops with severe bilateral hydrothorax (Figure 1), generalized skin edema, ascites and holosystolic tricuspid insufficiency. Doppler examination of the middle cerebral artery (MCA) revealed a peak systolic velocity (PSV) of 61 cm/s corresponding to 1.5 multiples of the median1 and normal pulsatility. Umbilical Doppler examination and further investigations (chromosomal analysis, TORCH serology and search for HbF cells in maternal blood) were unremarkable. At 28 + 4 weeks bilateral thoraco–amniotic shunts (Harrison fetal bladder stents) were inserted. Only a few hours after successful drainage and cardiac decompression ultrasound examination revealed an increase of MCA-PSV to 100 cm/s, indicating fetal anemia (Figure 2). Fetal blood sampling confirmed severe anemia (hemoglobin concentration, 5.8 g/dL) and intrauterine transfusion was performed. After the procedure MCA-PSV normalized to 67 cm/s and signs of hydrops started to regress. One week later a second transfusion of packed erythrocytes was necessary. At 30 + 3 weeks of gestation secondary Cesarean section was performed because of preterm rupture of membranes followed by labor. The female newborn (weight, 1610 g; Apgar scores 7, 9 and 10 at 1 min, 5 min and 10 min, respectively; hemoglobin concentration, 13 g/dL) was referred to the neonatal intensive care unit. She was discharged in healthy condition at 6 weeks of age and development at the age of 1.5 years appeared to be normal. Placental histology confirmed a chorioangioma of 115 × 95 mm. Bilateral severe hydrothorax and skin edema in a 28-week fetus. Middle cerebral artery Doppler velocimetry following shunt insertion for bilateral hydrothorax, showing a peak systolic velocity of 100 cm/s. In the presented case chorioangioma led to severe but masked anemia2, 3, which was correctly diagnosed by measuring MCA-PSV4, 5 only after drainage of the severe bilateral hydrothorax. A likely explanation is cardiac compression due to the hydrothorax, leading to impaired ventricular filling and low cardiac output, resulting in relatively normal MCA-PSV despite severe anemia. After successful drainage of fluid the intrathoracic pressure decreased and the cardiac tamponade disappeared, leading to increased ventricular filling, stroke volume and cardiac output which resulted in higher MCA-PSV, correctly indicating the presence of fetal anemia. The pathomechanism of fetal cardiac compression leading to diminished cardiac output and hydrops has been investigated in the context of different thoracic anomalies6, 7. Swast et al.8 measured diminished cardiac output in fetuses with congenital cystic adenomatoid malformation of the lung. Bigras et al.9 found that fetuses with hydrothorax had smaller dimensions of the left ventricle and the peak velocity of the ascending aorta was lower than that of the normal population. Both authors hypothesized that these findings may represent lower preload of the left ventricle caused by increased intrathoracic pressure. In addition, Van Mieghem et al.10 recently found decreased MCA-PSV in fetuses with prenatally diagnosed congenital diaphragmatic hernia. The authors' explanation was similar to our observation, with a smaller dimension of the left ventricle and decreased left ventricular output due to intrathoracic compression. In summary, elevated intrathoracic pressure may generate distinct alterations in fetal hemodynamics, which are likely to affect MCA-PSV. Therefore, evaluation of MCA-PSV to exclude associated fetal anemia should be interpreted with caution in the presence of bilateral hydrothorax. In these cases drainage of bilateral pleural effusions may normalize intrathoracic pressure and increase cardiac output. A. Hellmund*, C. Berg* , B. Rösing*, U. Gembruch*, A. Geipel*, * Department of Obstetrics and Prenatal Medicine, University Medical School Bonn, Bonn, Germany, Division of Prenatal Medicine and Gynecologic Sonography, University Medical School Cologne, Cologne, Germany