Unilateral rib agenesis: antenatal three-dimensional ultrasonographic detection of the rare congenital anomaly of ribs.

Congenital absence of ribs is a quite unusual entity that can present as an isolated finding or as a part of syndrome. Rib defects may be associated with maldevelopment of vertebral column or thoracic musculature. The congenital rib agenesis has been reported postnatally in a few case reports. We present a case of a primigravida referred for an anomaly scan at 20-weeks gestational age. On ultrasonography (USG), the fetus showed unilateral absence of left 5th-8th ribs with associated dysmorphism of other ipsilateral ribs, a right-sided lumbar supernumerary rib, and vertebral segmentation anomalies. The bony defects were well demonstrated by three-dimensional (3D) transabdominal USG. In our case, the unilateral rib agenesis was detected antenatally with no such report published earlier in the scientific literature.