CONGENITAL DISORDER

particularly in human beings, any abnormality of structure, deficiency of function, or disease that is present at the time of birth, whether caused by genetic transmission, accident, or infection. The more restrictive term birth defect denotes abnormalities present at birth apart from diseases, such as syphilis, that can attack the child before birth and damage structures already formed. Among infections possibly only rubella (German measles) affects the embryo during its period of development, producing true malformations. The unborn human infant acquires its essential form, and its organs and tissues are all laid down and defined, within the first eight weeks after conception. This is the period during which the child is often, but not always, described as an embryo; only afterward is it called a fetus. After the first eight weeks there is differentiation and growth, and some anomalies may arise, especially of the brain, eye, and inner ear; but all gross disturbances of form will already have occurred. The distinction between embryo and fetus is useful if it emphasizes the early period of development critical in the production of congenital defects. Many biochemical defects may not be manifest until the metabolism is separated at birth from that of the mother; all such defects, however, have their inception in an earlier failure to develop some enzyme system.