Bất thường của nhiễm sắc thể NST thường thường gây ra nhiều dị tật bẩm sinh và chậm phát triển tâm thần. Trẻ em với nhiều hơn một bất thường và chậm phát triển hoặc khuyết tật học tập vì vậy phải trải qua phân tích nhiễm sắc thể như là một phần của điều tra của họ. | 5 Common chromosomal disorders Abnormalities of the autosomal chromosomes generally cause multiple congenital malformations and mental retardation. Children with more than one physical abnormality and developmental delay or learning disability should therefore undergo chromosomal analysis as part of their investigation. Chromosomal disorders are incurable but most can be reliably detected by prenatal diagnostic techniques. Amniocentesis or chorionic villus sampling should be offered to women whose pregnancies are at increased risk - namely couples in whom one partner carries a balanced translocation women identified by biochemical screening for Down syndrome and couples who already have an affected child. Unfortunately when there is no history of previous abnormality the risk in many affected pregnancies cannot be predicted before the child is born. Down syndrome Down syndrome due to trisomy 21 is the commonest autosomal trisomy with an overall incidence in liveborn infants of between 1 in 650 and 1 in 800. Most conceptions with trisomy 21 do not survive to term. Two thirds of conceptions miscarry by mid-trimester and one third of the remainder subsequently die in utero before term. The survival rate for liveborn infants is surprisingly high with 85 surviving into their 50s. Congenital heart defects remain the major cause of early mortality but additional factors include other congenital malformations respiratory infections and the increased risk of leukaemia. An increased risk of Down syndrome may be identified prenatally by serum biochemical screening tests or by detection of abnormalities by ultrasound scanning. Features indicating an increased risk of Down syndrome include increased first trimester nuchal translucency or thickening structural heart defects and duodenal atresia. Less specific features include choroid plexus cysts short femori and humeri and echogenic bowel. In combination with other risk factors their presence indicates the need for diagnostic .
