The transverse section of the fetus urinary bladder is circular and, for volume estimation, an appropriate...

The purpose was to investigate the transverse shape of the fetal urinary bladder.

Using 3D ultrasound, an appropriate longitudinal image of fetal urinary bladders was chosen. Perpendicular sections were examined and the transverse bladder image with a minimum of shadows was selected, documented and transferred to a PC. The contour of the transverse bladder image was marked manually and recorded electronically. The mean of radii distances from the centre pixel to each border pixel outlining this bladder image was calculated for each fetus.

In the material comprising 20 fetuses, the range of mean radii was 2.6-18.4 mm and the SD 0.4 mm. The mean wall thickness of the bladders was 2.7 mm and the distance represented by one pixel for different selected depths was 0.3 mm on average.

In view of the current technical limitations, an aberration from the circular shape may not be discernible. For this reason, the transverse shape of the fetal urinary bladder can be regarded as a circle. This means that, for bladder volume estimation, only an appropriate longitudinal section is necessary.