Trisomy 18

Trisomy 18


Trisomy 18 is the second most common chromosome abnormality in newborns. The incidence is 1:3000-5000 liveborns and 80% of the cases are female. The majority of trisomy 18 cases are the result of meiotic non-disjunction in the egg and the condition is associated with advanced maternal age. Mosaicism occurs in about 10%. Most cases are aborted spontaneously, and 85% of the cases detected at 12 weeks gestation and 75% detected at 16 weeks gestation will be spontaneously aborted or stillborn. The life expectancy is very short with less than 10% living more than 1 year.


The clinical findings include low birth weight, hypertonia and characteristic facial features with microcephaly, dolichocephaly, prominent occiput, narrow palpebral fissures, hypoplasia of the malar areas, a small mouth, retrognathia, low set malformed ears. Brain abnormalities include hypoplasia or absence of the corpus callosum, Dandy-Walker malformation, absent septum pellucidum, periventricular heterotopia, short frontal lobe and abnormal gyri. The neck manifestations are redundant nuchal skin and thyroglossal cyst. The chest has a short sternum, widely spaced and hypoplastic nipples and eventration/hernia of the diaphragm. Cardiac abnormalities are common and include polyvalvular abnormalities, VSD, PDA, hypoplastic left heart, coarctation of the aorta, PDA and PFO. The abdomen shows inguinal and umbilical hernia, single umbilical artery. Gastrointestinal abnormalities include esophageal atresia, ectopic pancreas and anal atresia. Renal abnormalities including cystic kidneys, hypoplastic kidneys, and horseshoe kidneys and double ureters are common. Genital abnormalities include cryptorchidism, prominent clitoris and hypoplasia of the labia majora, bicornuate uterus and ovarian cyst. The pelvis is small and the limbs shows overlapping fingers, hypoplastic nails, club foot, prominent calcaneus, rocker-bottom feet.

Differential Diagnosis

Pena-Shokeir syndrome type I Trisomy 13

Sonographic Features

Intrauterine growth retardation


Non-immune hydrops fetalis


Choroid plexus cyst

Agenesis of the corpus callosum

Dandy-Walker malformation

Dandy-Walker variant

Absent septum pellucidum

Periventricular heterotopia

Enlarged cisterna magna


Cleft lip and/or palate

Low-set/abnormal ears

Cystic hygroma

Increased nuchal translucency/nuchal fold thickening

Diaphragmatic hernia

Cardiac abnormalities

Single umbilical artery


Umbilical cord cyst

Esophageal atresia

Echogenic gut

Renal hypoplasia

Horseshoe kidneys



Renal cysts

Ambiguous genitalia

Radial aplasia/hypoplasia

Overlapping fingers

Rocker-bottom feet

Short limbs


Associated Syndromes