Among liveborn children, trisomy 18 is the second most common autosomal trisomy after trisomy 21.
The disorder is characterized by severe psychomotor and growth retardation, microcephaly, microphthalmia, malformed ears, micrognathia or retrognathia, microstomia, distinctively clenched fingers, and other congenital malformations.  See the images below.
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Trisomy 18 severely affects all organ systems.
In translocations that result in partial trisomy or in cases of mosaic trisomy 18, clinical expression is less severe, and survival is usually longer.
Prevalence is approximately 1 in 6,000-8,000 live births.
At the time of first trimester screening, the incidence of trisomy 18 is 1 in 400, but due to high spontaneous loss, the birth prevalence is 1 in 6,500.
Approximately 95% of conceptuses with trisomy 18 die as embryos or fetuses; 5-10% of affected children survive beyond the first year of life.
For liveborn infants with trisomy 18, the estimated probability of survival to age 1 month was 38.6% and to age 1 year was 8.4%. Median survival time was 14.5 days (population based study).  Nonetheless, in a multistate study of 1113 children with trisomy 18, Meyer et al found a 5-year survival rate of 12.3%. In the study, gestational age had the greatest impact on mortality, while the lowest mortality rates were found among females and the children of non-Hispanic black mothers.  Long-term survival up to age 27 years has been reported. [6, 7, 8]
The high mortality rate in trisomy 18 is usually due to the presence of cardiac and renal malformations, feeding difficulties, sepsis, and apnea caused by CNS defects. Severe psychomotor and growth retardation are invariably present in those who survive beyond infancy.
Trisomy 18 has no racial predilection.
Approximately 80% of trisomy 18 cases occur in females. The preponderance of females with trisomy 18 among liveborn infants [4, 9] (sex ratio, 0.63) compared with fetuses with prenatal diagnoses (sex ratio, 0.90) indicates a prenatal selection against males with trisomy 18 after the time of amniocentesis. [10, 11, 12]
Trisomy 18 is detectable during the prenatal and newborn periods.
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