Objective To explore the feasibility of using transverse of posterior brain with ultrasound for observing the intracranial posterior brain structure of fetuses in 11～13

+6

gestational weeks.Methods Two thousand six hundred and ninety-five normal fetuses of 11～13

+6

gestational weeks were randomly selected to obtain the mid-sagittal and posterior transverse sections of the brain.The structure of the posterior brain was observed and the thickness of the intracranial transparent layer was measured to analyze the consistency of the thickness of the intracranial transparent layer detected by the two sections.Results Among the 2695 fetuses

2402 cases received mid-sagittal and posterior transverse sections of the brain（1 case showed abnormal posterior brain structure）

and 239 cases received only posterior transverse sections of the brain but no mid-sagittal sections.54 cases failed to receive any two kinds of sections of brain.The structure of "two high and three low echo cords" in normal fetal posterior brain was observed on sagittal section and transverse section.The values of intracranial translucency thickness measured in the two sections were in good agreement（the consistency bias was 0.0219

consistency boundaries was-0.2740-0.2874）.Conclusion The using of posterior transverse section in first trimester to observe fetal posterior brain structure can be a safe

rapid and effective method for screening fetal nervous system malformations.