Abstract: A total of 300 pregnant women in our hospital who underwent routine antenatal secondary obstetric ultrasound examination were selected, all of whom underwent ultra-high frequency pure wave ultrasound examination at 28-40 weeks in the third trimester. The results showed that the morphology of the third ventricle of 300 fetuses was divided into 14 cases (4.67%) with similar single-line sign, 265 cases with parallel line sign (88.33%), and 21 cases with "V" sign (7.00%). The width and anteroposterior diameter of the third ventricle were positively correlated with the gestational age and biparietal diameter of pregnant women (P<0.05). The 95% reference values of the width and anteroposterior diameters of the third ventricle of 300 pregnant women were 1.21-1.64 mm and 10.00-10.41 mm, respectively. It is suggested that the use of ultra-high frequency pure wave ultrasound to measure the third ventricle of the fetus in the third trimester of pregnancy is helpful to obtain clearer ventricle morphology and measurement data. It can also provide an important reference for clinical prenatal diagnosis and screening of central nervous system malformations.

Key words: ultra-high frequency pure wave ultrasound, third trimester, fetus, central nervous system malformation, third ventricle, prenatal screening