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KEY POINTS

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Key Points

  • Second most common autosomal trisomy in liveborn infants.

  • Eighty to 85% of cases result from full trisomy, 10% are mosaics, and 5% are due to translocation.

  • Incidence is about 1 in 3000 livebirths. Females are more likely to be born alive and survive longer.

  • Most affected fetuses have more than one sonographic abnormality. Most common sonographic abnormalities include IUGR, cardiac structural anomalies, choroid plexus cysts, central nervous system anomalies, and overlapping fingers/clenched hands.

  • Associated with low maternal serum screening levels of pregnancy-associated plasma protein A, estriol, α-fetoprotein, and β human chorionic gonadotropin.

  • Median postnatal survival for males is 1 to 2 months, and for females it is 9 to 10 months. The presence of a heart defect (surprisingly) does not affect postnatal survival time. About 5% to 10% of infants survive until their first birthday.

  • All long-term survivors are profoundly retarded. Despite this, there are increasing reports in the literature of aggressive postnatal treatment, including mechanical ventilation, cardiovascular drugs, parenteral nutrition, and surgical repair of congenital anomalies.

  • Reports exist of adults with mosaic trisomy 18 who have normal intelligence.

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CONDITION

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Trisomy 18 is a chromosomal abnormality that results from the presence of an extra copy of chromosome 18. It is the second most common autosomal trisomy in liveborn infants. The clinical features associated with the abnormality were first described by Edwards in 1960. The condition is also known as Edwards syndrome or trisomy E. Of patients with trisomy 18, 80% to 85% have a full extra copy of chromosome 18 in all of their cells, 10% have mosaicism, with a normal cell line in some of their cells, and 5% have the long arm of chromosome 18 translocated onto another chromosome (Hill, 1996).

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INCIDENCE

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The incidence of trisomy 18 varies from approximately 1 in 3000 to 1 in 7000 livebirths. The specific incidence of trisomy 18 in Leicestershire, England, was studied during the years 1980 through 1985. At that time the incidence was noted to be 1 in 3086 livebirths (Young et al., 1986). A less frequent incidence of 1 in 6806 livebirths was noted during a 10-year period in Denmark (Goldstein and Nielsen, 1988). The incidence of trisomy 18 mosaicism is approximately 1 in 70,000 livebirths (Bass et al., 1982). Interestingly, the sex ratio of fetuses and livebirths with trisomy 18 differs. Sex ratios are defined as the number of males divided by the number of females. In prenatal normal controls, the ratio is 1.07. The sex ratio for fetal cases of trisomy 18 is 0.90 and for livebirths it is 0.63. Therefore, a clear-cut differential natural selection against males with trisomy 18 exists after 16 weeks of gestation (Huether et al., 1996). Females with trisomy 18 are more likely to be born alive and survive longer than males (Rasmussen et al., 2003...

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