ObjectiveTo measure fetal pericardial fluid in low-risk second-trimester pregnancies and to evaluate outcome for those with measurements greater than 2 mm.MethodsFive hundred and six women were referred for sonography between 16 and 25 weeks' gestation for common obstetric indications (dating, fetal survey, and placental location) unrelated to an increased risk of anomalies. All cases were evaluated with two-dimensional and M-mode real-time ultrasonography with the use of a mechanical sector transducer. The maximum distance of the fetal hyposechoic cardiac rim was recorded. We reviewed maternal and infant charts for those with measurements greater than 2 mm.ResultsMedian (range) maternal age was 25 (15–42) years. Median gravidity and parity were two (1–4) and one (0–11), respectively. Median estimated gestational age was 20.4 (16.324.9) weeks. Fetal pericardial fluid was seen in 360 of 506 (71%) fetuses. Of these 360 fetuses, the mean distance (± 2 standard deviation) of the fetal hypoechoic cardiac rim was 1.20 mm ± 0.91 mm (95% confidence interval 1.15, 1.25). Among the 506 cases, the maximum measurement was 3 mm. Ten of the 506 (2%) cases had measurements greater than 2 mm. None of these ten fetuses had a cardiac structural abnormality or arrhythmia, and perinatal outcome was unremarkable.ConclusionDuring second-trimester fetal ultrasonographic examination, visualization of pericardial fluid up to 2mm in the fetus with current high-resolution technology is common and should not be regarded as pathologic.