Sparks TN, Lianoglou BR, Adami RR, et al. - Nonimmune hydrops fetalis (NIHF) is a fetal abnormality which is frequently lethal and has numerous genetic causes. Researchers here examined the extent to which exome sequencing can aid in its diagnosis. In this study, an evaluation was performed of a series of 127 consecutive unexplained cases of NIHF that were defined by the presence of fetal ascites, pleural or pericardial effusions, skin edema, cystic hygroma, raised nuchal translucency, or a combination of these conditions, were evaluated. In this large case series of fetuses with unexplained NIHF, one third of the cases exhibited diagnostic genetic variants, including those for disorders affecting the RAS–MAPK cell-signaling pathway (known as RASopathies) (30% of the genetic diagnoses); inborn errors of metabolism and musculoskeletal disorders (11% each); lymphatic, neurodevelopmental, cardiovascular, and hematologic disorders (8% each); and others.