Aim: Cerebellar hypoplasia, particularly, trisomy 18, represents a chromosomal abnormality. However, morphological assessment of the fetal cerebellum is yet to be established. The purpose of this study was to produce reference values for the fetal transverse cerebellar diameter (TCD), anteroposterior cerebellar diameter (APCD) and the ratio of APCD to TCD (APCD/TCD ratio) in normal fetuses, compared with cases of trisomy 18.
Methods: This retrospective study included 266 normal singleton pregnancies resulting in term delivery at our institution from 2012-2013. Patients had received ultrasound examinations in the second trimester. TCD and APCD measurements were performed and then APCD/TCD ratio was calculated. Consequently, three cases of trisomy 18 were compared with reference values.
Results: Gestational age correlated with TCD (rs = 0.766, P < 0.0001) and APCD (rs = 0.61, P < 0.0001), but not with APCD/TCD ratio (rs = 0.070, P = 0.252). The median APCD/TCD ratio was 0.52 and the standard deviation was 0.05. In cases of trisomy 18, all TCD measurements were located within normal ranges. In contrast, both the APCD and APCD/ TCD ratio displayed lower values than that of normal fetuses.
Conclusions: This study showed that not only TCD but also APCD correlated with gestational age, and APCD/TCD ratio indicated a fixed value (almost 0.5) in normal fetuses. Furthermore, APCD and APCD/TCD ratio were deemed useful indicators for cerebellar hypoplasia. In particular, the APCD/TCD ratio proved more convenient, without requiring individual evaluation of TCD and APCD.
Keywords: cerebellar diameter; cerebellar hypoplasia; fetal cerebellum; trisomy 18; ultrasonography.
© 2015 Japan Society of Obstetrics and Gynecology.