How can a genetic disorder be congenital without being inherited from a parent?

A genetic disorder can be congenital—meaning it exists at birth—without being inherited from a parent through several mechanisms:

• New Mutations: A mutation can occur for the first time in the male or female gamete (sperm or egg) or during the early development of the fetus. In these cases, the child possesses a genetic abnormality that the parents do not have.12
• Chromosomal Abnormalities: Many congenital disorders, such as Down syndrome, result from numerical chromosomal errors like trisomy. These typically arise from meiotic nondisjunction—the unequal division of chromosomes—during gamete formation. Because these are independent events, they are generally not inherited from parents.1234
• Epigenetic Defects: Disruptions in genomic imprinting can cause abnormal expression of maternal and paternal alleles, interfering with embryonic development even if the underlying DNA sequence is not mutated in the traditional sense.15

It is important to distinguish these genetic origins from non-genetic congenital diseases, which can be caused by direct environmental injury to the fetus, such as a viral disease like rubella contracted during pregnancy.2