Monosomy 18p Presenting With Holoprosencephaly and Increased Nuchal Translucency in the First Trimester

OBJECTIVE: The purpose of this series is to describe the first-trimester sonographic findings leading to the diagnosis of monosomy 18p. METHODS: The sonographic appearance of 2 cases of monosomy 18p, detected as part of first-trimester sonographic screening for aneuploidy at this institution, was reviewed. RESULTS: Increased nuchal translucency thickness, alobar holoprosencephaly, and facial defects were detected in both cases. Chromosomal analysis by chorionic villus sampling revealed de novo del(18p) in 1 case and monosomy 18p as the result of an 18q isochromosome in the other. CONCLUSIONS: Increased nuchal translucency thickness in association with brain and face anomalies on a first-trimester scan allowed the early diagnosis of monosomy 18p. Fetuses with holoprosencephaly can be readily identified in the first trimester by examining a cross section of the fetal head looking for the "butterfly" sign. Prenatal karyotyping provides important information regarding the etiology and risk of recurrence in these cases.