1) A birth defect is an abnormality present at birth. Also called a congenital defect, it can be caused by a genetic mutation, an unfavorable environment during pregnancy, or a combination of both. The effect of a birth defect can be mild, severe, or incompatible with life.

2) A physical or biochemical defect (as cleft palate, phenylketonuria, or Down syndrome) that is present at birth and may be inherited or environmentally induced.