Birth defect is an abnormality in the structure or function of the body that is present at birth. The term birth defect often refers to a body part with a structural abnormality, such as a clubfoot or cleft lip. However, some authorities also consider inherited diseases that don't produce a visible abnormality, such as phenylketonuria and sickle cell anemia, to be birth defects. Birth defects are also called congenital defects.