11 pairs of ribs is more commonly seen in otherwise normal individuals than associated with congenital disease. In the majority of cases the twelfth ribs are the absent ribs. In the case presented here no underlying congenital disease process was ever identified and therefore it was decided to be an incidental anatomic variant.

Studies have shown that 11 pairs of ribs are slightly more common in females ².

It has been estimated that 5-8% normal people have only 11 pairs of ribs. When an underlying condition is present, the commonest found is trisomy 21. In those with trisomy 21, one-third of patients are missing their twelfth ribs, other causes of 11 rib pairs are cleidocranial dysplasia and campomelic dysplasia ¹. 

According to one study which reviewed the chest radiographs of 30 infants with trisomy 21 and another 881 CXRs of normal infants, if 11 pairs of ribs are the sole abnormality on the chest radiograph then only 0.2% of the time will this be trisomy 21 ². All the other patients in this study, bar one, had a normal number of pairs of ribs, i.e. twelve.