Many families first notice 13q12.3 microdeletion syndrome in infancy or early childhood when a baby feeds poorly, grows more slowly than expected, or reaches milestones like sitting and walking later than peers. Doctors are often alerted by low muscle tone, distinctive facial features, or hearing concerns at early check-ups, and may find congenital differences such as heart or kidney anomalies. Genetic testing—prompted by these early clues—confirms the diagnosis, explaining the first signs of 13q12.3 microdeletion syndrome.