Agenesis of the ramus of the mandible (agnathia) is a rare disease that can be part of more than 100 genetic syndromes. From an embryological point of view, agenesis of the mandible is linked to defects in migration of neural crest cells that determine a developmental anomaly of the first arch, or mandibular arch, during the 4th week of embryogenesis (Jones KL. Smith’s recognizable patterns of human malformation, 5th edn. WB Saunders, London, 1997). Agnathia can be isolated or commonly associated with holoprosencephaly. Evidence from humans and animal model suggest that agnathia-holoprosencephaly represents a causally heterogeneous single developmental field defect (Pauli RM, Petterson JC, Arya S, Gilbert EF. Am J Med Genet 14:677–698, 1983). Agnathia may be usually recognized by ultrasound during the second trimester of pregnancy and may be misinterpreted as micrognathia (Turner GM, Twining P. Clin Radiol 47:389–395, 1993; Nicolaides KH, Salvesen DR, Snijders RJM, Gosden CM. Fetal Diagn Ther 8:1–9, 1993; Bromley B, Benacerraf BR. J Ultrasound Med 13:529–533, 1994). If micrognathia is the only sonographic finding identified, parents should be awarded for possible respiratory complications at birth, presence of facial clefting, and/or developmental delay (Vettraino I, Lee W, Bronsteen RA, Harper CE, Aughton D, Comstock CH. Obstet Gynecol 102:801–805, 2003). Chromosomal abnormalities have been detected in 66% of fetuses with micrognathia, in 77% with macroglossia, in 48% with cleft lip and palate, in 45% with severe hypotelorism or cyclops, and in 32% with nasal hypoplasia, proboscis, or single nostril (Nicolaides KH, Salvesen DR, Snijders RJM, Gosden CM. Fetal Diagn Ther 8:1–9, 1993; Brizot ML, Schultz R, Patroni LT, Lopes LM, Ambruster-Moraes E, Zugaib M. Prenat Diagn 21:672–675, 2001). Teoh and Meagher (Ultrasound Obstet Gynecol 21:616–618, 2003) have reported a first-trimester (13 weeks) diagnosis of micrognathia and receding chin by using transvaginal ultrasound, with later diagnostic confirmation at 19 weeks’ gestation. These findings were associated with complex cardiac defect and unilateral talipes equinovarus in a fetus with normal karyotype and with a definitive diagnosis of Pierre Robin syndrome. Sepulveda et al. (Ultrasound Obstet Gynecol 39:152–156, 2012) have demonstrated the value of the retronasal triangle view for detecting micrognathia in the first trimester. Absent visualization of the mandibular gap or failure to identify the mandible in coronal view is highly suggestive of micrognathia and should prompt a targeted ultrasound scan to assess for other anomalies. The role of RNT in the early prenatal diagnosis of orofacial clefts has been confirmed later by Tonni et al. (Fetal Diagn Ther 34:31–37, 2013).