Invasive prenatal testing, amniocentesis and chorionic villus sampling, has been used for over four decades to identify fetal genetic disorders. The most common test after obtaining fetal tissues is chromosome analysis, performed for a variety of medical indications including abnormal ultrasound findings, advanced maternal age and an abnormal screen for Down syndrome. About 2% of pregnancies in women over the age of 35 will show a chromosome abnormality, with trisomy 21 being the most common. In addition to Down syndrome, the most commonly observed trisomies are those of chromosomes 13 and 18. Numerical abnormalities of the sex chromosomes are also relatively common, as well as triploidy.
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