Utility Of Fetal Anteroposterior To Transverse Cerebellar Diameter Ratio To Exclude Cerebellar Hypoplasia In Trisomy 18

Aim: This study was aimed to determine reference ranges for fetal cerebellar hemisphere biometry, including the transverse cerebellar diameter (TCD), anteroposterior cerebellar diameter (APCD) and APCD/TCD ratio in normal fetuses. In addition, we investigated which parameter would be useful for cerebellar hypoplasia in trisomy 18.Methods: This retrospective study included 340 normal singleton pregnancies and 15 cases of trisomy 18, in all of which fetal cerebellar biometry was performed between 14 and 40 weeks of gestational age (GA). The TCD, APCD and APCD/TCD ratio were assessed ultrasonographically.Results: In normal fetuses, the TCD (r(s) = 0.876, P < 0.001) and APCD (r(s) = 0.791, P < 0.001) were strongly correlated with GA. However, the APCD/TCD ratio was not correlated with GA (r(s) = 0.058, P = 0.289), with median values of 0.52. Low TCD, APCD and APCD/TCD ratio values were detected in 53%, 100% and 100% of trisomy 18 cases, respectively. The median APCD/TCD ratio for trisomy 18 was 0.39 (range, 0.30-0.43), which was significantly lower than that of normal fetuses (P < 0.001). A cut-off APCD/TCD ratio of 0.44 served as a good predictor for trisomy 18 (sensitivity 100%, specificity 95.3% and negative predictive value 100%).Conclusion: This study shows that TCD and APCD are correlated with GA, while the APCD/TCD ratio is a fixed value throughout gestation. Using the APCD/TCD ratio to assess cerebellar hypoplasia in trisomy 18 is useful because it does not require the individual evaluation of the TCD and APCD.