Trisomy 21

Trisomy 21


Trisomy 21 (Down syndrome) is the most common chromosome abnormality and the most common cause of mental retardation in newborns. About 65-80% of the trisomy 21 conceptions result in a miscarriage. However, 40% of the pregnancies with trisomy 21 detected at 12 weeks and 30% of those detected at 16 weeks gestation will be lost. The incidence is 1:600-1:800 livebirths and the male/female ration is 1. There are no racial differences and there is a strong association with maternal age. Thus, the birth prevalence is only 0.9/1000 when the mother is less than 33 and 38/1000 when the mother is older than 44. Most cases are the result of non-disjunction in the first meiotic division of the egg . The phenotype is the result of duplication of region 21q22 and 95% of the cases are the result of an extra chromosome 21, 3% are the result of translocation mainly Robertsonian. Of the trisomy 21 cases with Robertsonian translocation, about 50% of the D/21 translocation and 10% of the G/21 translocation are inherited. In 2% of the cases, mosaicism is detected and most of these cases are the result of postzygotic loss of one of the chromosome # 21 in a trisomy 21 zygote. The most common clinical findings in newborns with Down syndrome are hypotonia, poor neonatal reflexes, hyperextensible joints, redundant nuchal skin, upslanting palpebral fissures, short ears, dysplastic pelvis, fifth finger clinodactyly and single palmar creases. Other common findings are short stature, mental retardation, brachycephaly, flat occiput, epicanthic folds, Brushfield spots, prominent tongue, short broad hands, and ”sandal gap” toes.


Cardiovascular abnormalities are detected in about 40% of the cases. Approximately 70% of these are AV canal. About 8% of the patients with Down syndrome have duodenal atresia. Their mental development shows an early rise in IQ with a plateau from age 2 to 5 years followed by gradual decline. The life expectancy is generally decreased. A survival of 88% at one year and 82% at 10 years were found. Congenital heart defects reduced survival to 72% and complete atrio-ventricular canal defects reduced the survival to 58% at 10 years. However, a trend towards improved survival was found with A-V canal when surgically treated. Other causes of the increased mortality are infections and malignancies.

Differential Diagnosis

Cystic hygroma can be associated with other chromosome abnormalities including trisomy 13 and 18. It is also associated with single gene disorders including Turner syndrome autosomal dominant cystic hygroma/pterygium colli, achondroplasia, lethal multiple pterygium syndrome, Roberts syndrome, Cumming syndrome, Cowchock syndrome and achondrogenesis type II among others. Short stature can be seen in Turner syndrome, familial short stature, growth hormone deficiency, hypothyroidism, multiple pterygium syndrome, prenatal exposure to teratogens such as alcohol and due to chronic condition. Lymphoedema can be seen in Milroy syndrome, lymphoedema with distichiasis, Hennekam syndrome, and lymphoedema with intestinal lymphangiectasis.

Sonographic Features



Flat facial profile

Small ears

Increased nuchal thickening and nuchal translucency

Cystic Hygroma

Cardiac abnormalities (especially AV canal but also isolated ASD, VSD, tetralogy of Fallot and hypoplastic left heart)

Duodenal atresia presenting as ”double bubble”


Short humeri and femora


Bladder neck obstruction

Papillary muscle calcifications

Short middle phalanx of the 5th finger

Isolated pleural effusion

Echogenic bowel ”Sandal” gap

Mild cerebral ventriculomegaly

Dandy-Walker malformation

Dandy-Walker variant

Associated Syndromes