Fetal Sacral Ratio: A Novel Method for Prenatal Sonographic Assessment of Sacral Abnormalities

Objectives: The sacral ratio (SR) was described as a postnatal X-ray-based method to detect sacral abnormalities and predict functional prognosis for fecal continence in children with anorectal malformations (ARMs). The present study aimed to describe a novel method of assessing sonographic fetal sacral ratio (f-SR) in a normal population of fetuses.

Methods: Sixty three-dimensional (3D) ultrasound reconstruction images of the sacrum obtained from routine low-risk scans performed between 21 and 26 weeks of gestation served for measurement. The f-SR was calculated in a coronal view as the ratio between lines drawn at the upper and lower levels of the iliac bone and the 5th sacral vertebra. Bland-Altman plots assessed the inter- and intrareader variabilities of measurements.

Results: The f-SR in the normal population of fetuses was 0.913 (±0.094). During the study period, three cases with ARM were examined and had a mean f-SR of 0.55. There was good repeatability of measurements and between readers' agreement.

Conclusions: The present study introduces a novel prenatal sonographic f-SR that can be reliably calculated on prenatal 3D ultrasound with good reliability and reproducibility. Future research will identify the clinical significance of f-SR abnormalities in ARM and their long-term impact on continence.