Better known as Down's Syndrome, mongolism is a congenital abnormality in which the baby is born mentally retarded, with an average IQ of 50 - 60 and with physical deformation of the face (low set ears, sloping forehead, mongoloid eyes), and shortness of stature. Down's syndrome is caused by an extra chromosome 21 present in the G group. In a small percentage of cases it can be caused by the switching of chromosomes 14 or 15 in the D group or chromosome 21 or 22. Down's syndrome occurs in about 1 in 650 births, and is more commonly associated with children of mother's over the age of 35. Incidence in children of mothers over the age of 40 can be as high as 1 in 80. Down's syndrome can be detected prenatally through amniocentesis. The mortality rate is high within the first few years. It is also known as trisomy 21 or trisomy G syndrome.