Purpose: To establish that incorrect scanning of the fetal posterior fossa may falsely create the appearance of a mega-cisterna magna (MCM) or Dandy-Walker variant (DWV).
Materials and methods: After routine sonography, 100 consecutive fetuses (gestational age range, 25-40 weeks) without abnormality underwent additional posterior fossa scanning in an attempt to create the appearance of an MCM (anteroposterior diameter larger than 10 mm) or DWV. Sonograms were obtained in a plane inferior to or angled more coronally than the routine axial plane.
Results: The posterior fossa was depicted in 93 fetuses. Pseudo-MCM occurred in 35 (38%) of 93 fetuses: 12 (30%) of 40 fetuses aged 25.0-29.9 weeks, 11 (50%) of 22 fetuses aged 30.0-34.9 weeks, and 12 (39%) of 31 fetuses aged 35.0-40.0 weeks. Pseudo-DWV occurred in 40 (43%) of 93 fetuses: 20 (50%) of 40 fetuses aged 25.0-29.9 weeks, 12 (55%) of 22 fetuses aged 30.0-34.9 weeks, and eight (26%) of 31 fetuses aged 35.0-40.0 weeks.
Conclusion: Sonography of the posterior fossa in an angled semi-coronal plane should be avoided because it may create an appearance that mimics an abnormality.