Pregnancy

These are causes by errors in the number or structure of chromosomes and the risk increases accordingly to maternal age. A chromosomal anomaly is present in 0.5% of newborns, in 5% of stillborns and in 50% of early miscarriages.
The most common are:

• Trisomy 21 or Down syndrome: there are three 21st chromosomes instead of two. Children with Down syndrome appear to have mental retardation and skeletal disorders, as well as cardiovascular anomalies (1/600).
• Trisomy 18: triplication of 18th chromosome.  Severe mental retardation and several anomalies cut down the survival rate up to one year old at less than 10% (1/6000).
• Trisomy 13: triplication of 13th chromosome. As in trisomy 18, less than 10% survive up to one year of life (1/10000) because of severe anomalies and mental retardation.
• Turner syndrome: one of the two X chromosomes that normally a female infant has is absent. Cardiovascular malformations, kidney abnormalities and infertility are common signs. The rate of that syndrome is 1/2500.
• Klinefelter syndrome: males that have one extra chromosome, XXY. Infertility, learning impairment, height (1/500).