To investigate the capability of tomography ultrasound (TUI) in evaluating the development of the normal anorectal structure to provide a reference for prenatal diagnosis of imperforate anus.

From June 2017 to July 2018, 315 singleton pregnant women at 19-40 weeks of gestational age were selected from Beijing Tiantan Hospital, and they ranged in age from 23 to 40 years, with an average age of 29±3 years. The TUI technique was applied to measure the fetal anal sphincter diameter (ASD), anal canal diameter (ACD), and anal canal length (ACL). The correlation between ASD, ACD, or ACL and gestational age was analyzed by Pearson correlation analysis, and the regression equation and curve were established. The intraclass correlation coefficient (ICC) and 95% confidence interval (CI) were used to evaluate the repeatability of the measurement of ASD, ACD, and ACL.

The display rate of fetal anorectum in all fetuses by the TUI technique was 97.8% (308/315). Pearson correlation analysis showed that the normal fetal ASD, ACD, and ACL were positively correlated with the gestational age of the fetus, and the regression equations are: ASD, Y=-0.001X²+ 0.421X-1.726 (r=0.83, P<0.01); ACD, Y=0.418X-4.207 (r=0.89, P<0.01); and ACL, Y=0.526X-1.062 (r=0.87, P<0.01).

TUI for assessment of fetal anorectum is feasible. This method can be applied to improve the display rate of fetal anorectum.

Tomography ultrasound imaging; Anorectal malformation; Anal atresia; Anal sphincter; Rectum; Anal canal