Abstract: Objective To establish the normal reference ranges of transabdominal ultrasound measurements of the posterior
fossa structure in fetuses at 11 to 13+ 6 gestational weeks and explore their clinical value in screening open spina bifida (OSB).
Methods Between January, 2013 and September, 541 randomly selected normal fetuses underwent nuchal translucency at the
gestational age 11 to 13+6 weeks. The parameters of the posterior fossa were measured in mid-sagittal view of the fetal face and
the axial view of the transverse cerebellum insonated through the anterior fontanel by transabdominal ultrasound to establish
the normal reference ranges. The measurements were obtained from 3 fetuses with OSB for comparison with the reference
ranges. Results In normal fetuses, the parameters of the posterior fossa measured in the two views showed no significant
differences (P>0.05). Two high echogenic lines were observed in normal fetuses, as compared with one in fetuses with OSB
representing the posterior border of the brain stem and the anterior border of the fourth ventricle. The line between the
posterior border of the fourth ventricle and the anterior border of the cisterna magna was not displayed in fetuses with OSB.
The anteroposterior diameters of the brain stem, the fourth ventricle, and cisterna magna all increased in positive correlation
with the crown-lump length in normal fetuses. In the 3 OSB fetuses, the anteroposterior diameter of the brain stem exceeded
the 95th percentile and the anteroposterior diameter of fourth ventrical-cisterner magena was below the 5th percentile of the
reference range for CRL; the brain stem to fourth ventrical-cisterner magena anteroposterior diameter ratio was increased to
above 1. Conclusion The established normal reference ranges of the parameters of fetal posterior fossa may provide assistance
in early OSB detection. The absence of the posterior border of the fourth ventricle and the anterior border of the cisterna
magna and a brainstem to fourth ventrical-cisterner magena anteroposterior diameter ratio greater than 1 can be indicative of
OSB at 11 to 13+6 gestational weeks.