Prenatal Incidence and Clinical Value of the Absence of 12th Ribs in a Population in a Setting of Detailed Second-Trimester Sonography.

Objectives: We aimed to determine the incidence of the absence of the (last) 12th ribs in a population in a setting of detailed 2nd-trimester sonography using three-dimensional (3D) ultrasound and to assess whether or not this may be related to chromosomal aneuploidies.

Methods: Prospectively, we counted fetal ribs for the absence of the (last) 12th ribs in singleton pregnancies of women who presented to our clinic for detailed 2nd-trimester sonography. The assessment was carried out using 3D ultrasound. Volume data sets were acquired with the 3D skeleton mode using the maximum intensity with X-ray-weighted rendering. If the 3D skeleton mode was not sufficient, volume contrast imaging with the OmniView bone mode was used.

Results: The fetal ribs could be visualized in 97.01% of the 1,943 fetuses examined between 20 and 23 weeks' gestation. Timing the examination at 21, 22, or 23 weeks was found to be more successful than conducting it at 20 weeks. Twelfth ribs were found to be absent in 33 fetuses; 16 fetuses had 11 ribs unilaterally and 17 had so bilaterally. None of them had chromosomal abnormalities. Associated anomalies were present in 6 fetuses (18.2%); 2 of the anomalies were major and 4 minor.

Conclusion: The incidence of absent 12th ribs in this mixed population was 1.75%. In the absence of additional anomalies, the prognosis is favorable.