Objective To investigate the role of vaginal three-dimensional ultrasound measurement of interthalamic adhesion in the diagnosis of fetal midbrain aqueduct obstruction hydrocephalus. Methods A total of 60 pregnant women with fetal ventriculomegaly who were admitted to our hospital from January 2021 to April 2022 were selected, according to the results of transabdominal color Doppler ultrasound, fetuses with lateral ventricle dilatation were divided into mild lateral ventricle dilatation group and severe lateral ventricle dilatation group, and 43 healthy pregnant women who underwent physical examination during the same period were selected as the control group. All pregnant women underwent vaginal three-dimensional ultrasonography, to compare the differences of ultrasound indexes between the two groups and analyze their correlation with lateral ventricle dilatation. Results The fetal C, CD, and WA of the fetuses in the ventricle expansion group were significantly higher than those in the healthy control group (P<0.05), and APD, ITAD and ITAD / WA were significantly lower than that of healthy control fetuses (P <0.05) , Fetal C, CD and WA were significantly higher in the severe lateral ventricle expansion group than in the mild lateral ventricular expansion group (P<0.05), APD, ITAD and ITAD/WA were significantly lower than those in mild lateral ventricle dilatation group (P<0.05). The results of Pearson correlation analysis showed that lateral ventricle expansion was positively associated with fetal C, CD, and WA(P<0.05), and lateral ventricle expansion was negatively correlated with fetal APD, ITAD, ITAD/WA (P<0.05). Conclusion Transvaginal three-dimensional ultrasound can diagnose obstructive hydrocephalus with thalamic adhesions in the midbrain aqueduct of fetus.