Down syndrome (DS) is a condition caused by the presence of an extra chromosome (#21) at the time of conception. In 90% of cases, it is the result of nondisjunction during meiosis (Trisomy 21), which is usually a sporadic occurrence. In 4-5% of DS affected individuals a chromosomal imbalance is the cause, inherited from a parent who has a balanced karyotype (i.e. Robertsonian translocation). A small proportion of affected individuals, < 1%, are diagnosed with mosaic DS.
Note: ACMG provides healthcare professionals with open access ‘ACT Sheets’ to guide next steps following a positive NIPS report for trisomy 21 (see ‘Learn More – Primary Sources’ below)
ACOG Practice Bulletin 226: Screening for Fetal Chromosomal Abnormalities
ACMG: Noninvasive Prenatal Screening via Cell-Free DNA [Trisomy 21: Positive Cell Free DNA Screen]
CDC: Facts about Down syndrome
lettercase.org: Understanding a Down syndrome Diagnosis
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