Diagnosis of Fetal Posterior Fossa Malformations in High-Risk Pregnancies at 12–14 Gestational Weeks by Transvaginal Ultrasound Examination

<b><i>Objective:</i></b> To demonstrate sonographic detectable abnormalities of the posterior fossa in fetuses with a crown-rump length of 45–84 mm in high-risk pregnancies. <b><i>Methods:</i></b> This was a prospective, observational study including 47 fetuses with known outcome, whose mothers attended our centers for first trimester tests and showed an abnormal first trimester ultrasound scan. In these fetuses, we examined transvaginal acquired three-dimensional volume blocks for abnormalities of the fetal posterior fossa. In these fetuses, the measurements of the cerebellar vermis (VE) and of the anterior membranous area (AMA) were compared with published reference ranges. <b><i>Results:</i></b> There were 8 fetuses with a ver­mian length below the 5th centile. In 5 of these fetuses, the AMA was also elongated and in 4 of these fetuses, pathology was confirmed. One case with a normal vermian length and normal AMA had a hypoplastic VE later on in the confirmatory test. <b><i>Conclusion:</i></b> Pathology of the posterior fossa can be directly diagnosed by assessing the VE and the AMA at the first trimester scan by examining transvaginal acquired volume blocks.