Fetal Posterior Fossa Anomalies: Diagnosis-Specific Ultrasound–MRI Concordance and Divergent Perinatal Outcomes

Background and Objectives: We aimed to examine the demographic characteristics, prenatal imaging findings, genetic evaluation results, and postnatal outcomes of fetuses with posterior fossa anomalies (PFAs), and also to evaluate the diagnostic concordance between prenatal ultrasonography (USG) and fetal magnetic resonance imaging (MRI). Materials and Methods: This descriptive study analyzed singleton pregnancies referred to Izmir City Hospital and Tepecik Training and Research Hospital for suspected fetal PFA between 2016 and 2024. Data including USG findings, fetal MRI reports, genetic results, and perinatal outcomes were extracted from institutional records. Diagnostic concordance between USG and MRI was statistically assessed using the kappa coefficient. Results: Out of 152 fetuses with suspected PFA on USG, 116 underwent fetal MRI. Following the exclusion of normal MRI findings (n = 23), the final cohort comprised 93 fetuses with confirmed PFA. Mega cisterna magna (MCM) was the most prevalent diagnosis (54.8%), followed by cerebellar hypoplasia (CH) (15.1%) and Dandy–Walker malformation (DWM) (10.8%). A moderate-to-good diagnostic concordance was observed between USG and MRI (kappa = 0.640, p < 0.001). Significant differences were noted across MRI groups regarding gestational age at diagnosis (p < 0.001) and birth weight, which was notably lower in CH compared to MCM (p = 0.004). Clinical outcomes varied significantly by diagnosis (p < 0.001); while 74.5% of MCM cases showed normal development, adverse outcomes predominated in CH and Walker–Warburg syndrome. Conclusions: Fetal PFAs are a heterogeneous group of anomalies with different diagnostic and variable prognostic profiles. Fetal MRI improves anatomical classification and provides clinically significant contributions to prenatal counseling and perinatal management.