Abstract:Objective To determine the reference range of ultrasound measurement for normal fetal cerebellar hemisphere,including transverse cerebellar diameter (TCD),anteroposterior cerebellar diameter (APCD) and APCD/TCD ratio to analyze the diagnostic value of each parameter for cerebellar dysplasia. Method A total of 340 singleton pregnancies and 52 fetuses with trisomy 18 from April 2009 to January 2018 were retrospectively analyzed.Cerebellar measurements with ultrasonography were performed in all fetuses between 14 and 40 weeks of gestational age.The determined parameters included TCD,APCD and APCD/TCD ratio. Results Spearman correlation analysis showed that there was a strong correlation between TCD and gestational age (r=0.771,P<0.01),followed by APCD and gestational age (r=0.624,P<0.01),while there was no significant correlation between APCD/TCD ratio and gestational age (r=0.068,P=0.249).In 52 fetuses with trisomy 18,the median values of APCD,APCD/TCD ratio and TCD were 11.8 mm( 5.2-17.7 mm),0.39 ( 0.29-0.54),and 30.2 mm( 13.3-43.4 mm),respectively.APCD in 40 cases and APCD/TCD ratio in 48 cases were lower than those the reference range of normal fetuses.When the threshold of APCD/TCD ratio was set to 0.43,the sensitivity,specificity and area under the curve (AUC) for the diagnosis of trisomy 18 were 92.6%,72.4% and 0.78,respectively. Conclusion APCD/TCD ratio of trisomy 18 fetus is lower than that of normal fetus.It is more convenient and effective to identify the low APCD/TCD ratio than to evaluate TCD and APCD alone in the diagnosis of trisomy 18,and APCD/TCD ratio has a relatively fixed threshold throughout pregnancy (less than 0.43).