Diagnosing failure to thrive: 22q11.2 deletion syndrome in identical twins.

22q11.2 deletion syndrome (22q11.2DS) presents with a wide range of clinical manifestations, posing a diagnostic challenge. When cardinal characteristics, such as conotruncal cardiac anomalies or immunodeficiency, are absent, genetic testing may be delayed, postponing interventions to minimise developmental delay. We present a case of monochorionic monoamniotic identical twins diagnosed with de novo 22q11.2DS around 3 months of life, who lacked major characteristics of the disease. Both infants were admitted and discharged multiple times with consistent concerns for failure to thrive (FTT) and aspiration. Twin B was incidentally found to have asymptomatic hypocalcaemia and hypoparathyroidism, leading to a 22q11.2DS diagnosis. Twin A had no symptoms other than FTT and aspiration. This case encourages considering 22q11.2DS in the differential diagnosis for FTT, even when classic symptoms are absent or delayed in presentation. Early recognition allows for timely intervention and better outcomes.