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Down syndrome

Inheritance patterns and prenatal diagnosis

Inheritance patterns
Most cases of Down syndrome are sporadic but there is a small risk of recurrence in further pregnancies. The incidence of Down syndrome is related to maternal age. The older the mother the higher the risk of an affected child, though the majority of children with Down syndrome are born to younger mothers.

However, in a minority of cases (three to four per cent) a mother or father may have a balanced translocation of chromosome 21. In these cases the condition is inheritable. Genetic counselling should be sought in all cases.

An ante-natal screening test, the 'Triple Test,' is sometimes used. It involves taking a small sample of blood from the mother, but it is not a definitive test; it is a screening process, the aim of which is to calculate whether the likelihood of having a Down syndrome fetus is sufficiently high to warrant the performance of an amniocentesis. This screening test has a rather high false positive rate, and it is only the amniocentesis which provides a definitive answer.

Prenatal diagnosis
Amniocentesis is usually offered at sixteen weeks. Chorionic villus sampling is usually available at ten to twelve weeks. Fetal scans for 'neck translucency' are also used.

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Medical text written November 1991 by Contact a Family. Approved November 1991 by Professor M Patton, Professor of Medical Genetics, St Georges Hospital Medical School, London, UK and Dr J E Wraith, Consultant Paediatrician, Royal Manchester Children's Hospital, Manchester, UK. Last reviewed August 2004 by Professor Ben Sacks, Down Syndrome Educational Trust, Portsmouth, UK. Additional genetic information provided June 2005 by Dr L Devlin, Senior Registrar in Genetics, Belfast City Hospital, Belfast, UK.

 

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