Congenital means any trait or condition that exists from birth. A congenital disorder may be due to faulty development, infection, or the effects of the mother's exposure to some drugs or other toxic substances during pregnancy. Important examples of congenital disorders include spina bifida, with incomplete development of the spine, and congenital herpes, acquired from the mother during the passage through the birth canal.
Congenital causes of disorders are traditionally considered first for they act within the womb and cause disorders which are usually obvious at birth. Hare-lip, cleft-palate and club-foot are examples of common congenital disorders. Down syndrome, deformities of the heart (see congenital heart defects), some of which are responsible for "blue babies", and abnormalities of the nervous system associated with spastic paraplegia are others. As a class they are caused either by some fault in the chromosomal structure of the fertilized egg or by damage inflicted on the developing embryo in the womb – it is not always easy to decide which.
A developing embryo in the womb can be damaged by diseases contracted by the mother. German measles contracted during the early months of pregnancy, for example, can cause abnormalities of heart and ears (and sometimes of mentality and sight). Other congenital diseases are caused by drugs during pregnancy, such as the deformed babies that were born to mothers who had taken thalidomide. Smoking by a pregnant woman may also retard the normal growth of the fetus in the womb.
The disorders that are transmitted from the parents to the child from generation to generation are known as heredity disorders. Perhaps the most well-known is hemophilia, a disorder in which the clotting mechanism of the blood is deranged so that even trivial injury is followed by prolonged bleeding. Another heredity condition is phenylketonuria – in which there is an absence of enzymes that metabolize certain toxic phenyls, Untreated, it leads to severe mental retardation, but if detected soon after birth it can be dealt with quite successfully by a diet that is low in the amino acids that give rise to phenyls in the bloodstream.