Achondroplasia suspected on prenatal ultrasound and confirmed postnatally: clinical, radiological, and genetic correlation

Abstract Achondroplasia is the most common skeletal dysplasia, caused by activating mutations in the FGFR3 gene—most frequently c.1138G>A (p.Gly380Arg)—and is associated with rhizomelic disproportionate short stature. We report a newborn with strong prenatal suspicion based on third-trimester ultrasound findings, including marked rhizomelic shortening, an increased proximal metaphyseal–diaphyseal femoral angle, and the “collar hoop” sign. Notably, first- and second-trimester evaluations were unremarkable, with characteristic features becoming evident only in late gestation. The diagnosis was confirmed postnatally through clinical examination, radiographic findings, and identification of a de novo FGFR3 G380R mutation. This case highlights the diagnostic value of systematic third-trimester assessment when earlier studies are inconclusive and underscores the importance of integrating specific late-gestation ultrasonographic markers with postnatal clinical and molecular correlation. Careful evaluation of associated skeletal and craniofacial features is essential to refine the differential diagnosis in cases of late-onset femoral shortening. Detailed imaging can guide timely counselling and perinatal planning, particularly in settings where immediate molecular testing is not available.