Structural abnormality (e.g., agenesis ), functional problem (e.g., cystic fibrosis , phenylketonuria ), or disease present at birth.
Almost all are due to genetic factors (inherited or spontaneous {{link=mutation">mutation s, chromosomal disorder s), environmental influences during pregnancy ( rubella or other maternal factors, exposure to toxins or radiation), or both. The most sensitive period is the first eight weeks after conception, during which time the human embryo is essentially formed. Major congenital malformations form during this period. Some inherited disorders result from simple Mendelian dominance or recessiveness . Others may involve multiple genes. Chromosomal disorders are rare, because few affected fetuses survive to be born. Environmental influences may affect only one of a pair of identical twins. At least 30 significant defects probably occur per 1000 births. See also birth defect ; Down syndrome .