Birth defects, also known as congenital malformations, are structural, functional or metabolic disorders that are present at birth. They can be caused by genetic or environmental factors and occur during critical periods of development. The document categorizes birth defects into malformations, disruptions, deformations, and chromosomal aberrations. It provides examples of specific birth defects like spina bifida, phocomelia, anencephaly, and cleft palate. Causes of birth defects discussed include chemicals, infections, radiation, heavy metals and genetic factors.