Evaluation of Bi-Iliac Distance and Timing of Ossification of Sacrum by Sonography in the Second Trimester of Pregnancy

The sacral region in the fetus can be involved in many pathologies such as spina bifida as the most common spinal abnormality and caudal regression syndrome. On this basis, knowledge about the temporal sequences of sacral ossification centers appearances and other characteristics including bi-iliac distance with newer higher resolution ultrasound technique may be helpful in defining pathologies involving the sacral region and fetal skeletal maturation.

The goal of this research was to evaluate the timing of ossification of sacral vertebrae and bi-iliac distance as a guidance for clinicians to make an early and reliable diagnosis of sacral region anomalies such as caudal regression syndrome.

This cross-sectional study was done on 315 normal pregnancies at 14 to 25 weeks of gestational age. The sacral region was evaluated in spine up position, on a coronal plane. By changing the orientation of the probe between sagittal and coronal planes and continuous observation, presence of the ossification center could be confirmed. Then, the level of the central and neural arch centers was recorded in every gestational age.

Ossification happened in a predictable caudal direction pattern. S1 and body of S2 were visualized in all cases at 15 - 17 weeks + 6 days and all fetuses with 17 weeks of gestational age. The body of S3 was detected in all cases at 17 weeks and its arch at 20 weeks. Studying all fetuses at 18 weeks of gestation shows sacral ossification in the body and arch of S4. During the study period we had two cases with sirenomelia and three cases with sacral agenesis and we have explained their sacral region findings.

By using recent high-resolution ultrasound machines, sacral vertebral ossification centers could be identified in its early gestational age, which could be helpful for earlier detection of sacral pathologies.