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Trisomy 21 is a chromosomal abnormality, characterised by the presence of a third (partial or total) copy of chromosome 21. Although trisomy 21 is not a rare anomaly, the prevalence in the general population has decreased significantly as a result of routine prenatal testing. The prevalence at birth in France is currently estimated at 1/2 000 live newborns. Variable and often light intellectual deficiency, almost constant muscular hypotonia and joint laxity are usual consequences, often associated with morphological signs and risks of complications, which justify an adapted follow-up. Morphological features (upslanting palpebral fissures, epicanthus, flat neck, round face, small nose, bilateral single palmar crease) can be mild and do not constitute a hallmark of the condition. The main potential malformations and complications include: heart defects (atrio-ventricular canal), digestive malformations (duodenal atresia), congenital cataract, small size, Hirschsprung disease, West syndrome (see these terms), seizures, leukaemia, sleep apnea, sensory deficiencies, auto-immune and endocrine pathologies (hypothyroidism, gluten intolerance, diabetes, alopecia), earlier aging and Alzheimer disease. In 95% of the cases, trisomy 21 is ``free`` (i. e., the extra chromosome is due to an accidental non-disjunction during meiosis) and homogeneous; it is ``free`` and mosaic in 2-3% of the cases. Finally, in 2-3% of the cases, the supernumerary chromosome 21 or portion of chromosome 21 is integrated to another chromosome (translocation of a chromosome 21 to another chromosome). The diagnosis can be made based on the karyotype, which allows the exclusion of Zellweger syndrome, 9qter deletion (see these terms) or other chromosomal abnormalities. For the parents of a child affected by free trisomy 21, the recurrence risk is only slightly modified (1% until the age of 40 years, linked to maternal age afterwards). In case of Down syndrome caused by translocation, the risk is raised only if one of the parents has a balanced rearrangement. For a person with Down syndrome, the risk of transmitting the disease to the descendants is 1/3. In 70-75% of foetuses with Down syndrome, increased nuchal translucency can be seen on first-trimester ultrasonography. On the second-trimester examination, malformations (essentially heart and digestive) are present in 60% of the cases, and can be associated with minor morphological signs. Prenatal diagnosis can be confirmed by foetal karyotype on amniocentesis or chorionic villous sampling. Early educative program is essential. A well-adapted program, including re-education, schooling and social aspects should be proposed, aiming at obtaining the best possible personal fulfilment and integration in society, most of the time in an ordinary environment. An adapted medical follow-up is also useful, because of an increased risk of some disorders. It can be necessary to maintain some support in the adult age, including re-education. Median life expectancy is now above the age of 50 years.
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