To describe an atypical prenatal presentation of RAF1 -related Noonan syndrome (NS) without detectable cardiac abnormalities and to highlight sonographic features that may prompt early genetic evaluation.

We report a fetus with a de novo pathogenic RAF1 variant (c.770C > T, p.Ser257Leu) identified by exome sequencing following normal karyotype and chromosomal microarray analysis. Prenatal imaging demonstrated persistent increased nuchal translucency and nuchal fold thickness, marked polyhydramnios, disproportionate growth with enlarged head and abdominal circumferences but normal femur length, and abnormal thickening of the thigh soft tissue. Mild central nervous system findings were also noted. Serial fetal echocardiography revealed no structural or functional cardiac abnormalities.

This case suggests that detectable prenatal cardiac abnormalities may be absent in RAF1 -related NS, even in the presence of a pathogenic RAF1 variant. Disproportionate growth and abnormal soft tissue distribution may represent potential prenatal clues to RAF1 -related NS. When conventional chromosomal testing is unrevealing, RASopathies may be considered in the differential diagnosis, and ES may facilitate molecular diagnosis.