Down's syndrome, also known as trisomy 21, is a relatively common (1 in 1000 births) congenital disorder due to a chromosomal abnormality, usually of chromosome 21. Normally, a fertilized egg has 23 pairs of chromosomes. In most people with Down's syndrome, there is an extra copy of chromosome 21 (also called trisomy 21 because there are three copies of this chromosome instead of two), which changes the body's and brain's normal development. It causes a characteristic facial appearance (resembling that of a Mongolian), hand shape and skin patterns; mental retardation; and delayed growth. Congenital diseases of the heart and gastrointestinal tract are common, as is cataract. Down's syndrome is more common in babies born to mothers aged over 40 years. It was first described by the British physician John Down (1828–96).