A structural or functional abnormality present at birth, more sensitively termed a congenital anomaly since the word defect can feel stigmatizing to affected families. Causes include genetic mutations, chromosomal conditions such as Down syndrome, maternal infections, certain medications, and nutritional factors, most famously folate deficiency’s role in neural tube defects. The folate story is nutrition’s great preventive triumph: periconceptional folic acid supplementation and flour fortification have dramatically reduced spina bifida and anencephaly worldwide. Other defects link to maternal diabetes, alcohol, and rubella, each preventable through care before and during pregnancy. Surveillance registries track their occurrence to detect emerging threats, making birth defect monitoring a quiet but vital public health function.