GENERAL FEATURES OF TRISOMY 18

• Incidence
• Related to the stage of investigation.
• Table of age specific risk for trisomies 13, 18 and 21.
• Median life expectancy 5 days (range = 1 hour to 18 months) (1).
• Maternal age distribution is bimodal.
• Peaks at 25-29 years and 35-39 years.
• Workers have suggested that there may be a tendency to non-disjunction in some individuals that is independent of maternal age (2,3).
• 80-85% have an additional chromosome 18.
  10% are mosaics.
  5% result from a translocation.
• Neonates with mosaic Trisomy 18 are less severely affected than neonates with an extra chromosome 18 (4).
• Duplication of the short arm of chromosome 18 is not associated with either an abnormal phenotype or mental retardation. It appears that the long arm of chromosome 18 is responsible for the phenotypic characteristics (4).
• Levels of free beta hCG and alpha fetoprotein in trisomy 18.

AFP Level
Free Beta Level		Low	Within Normative Range	High
	Low	Trisomy 18 Fetal Demise Chromosomal Abnormalities Perinatal  Complications*		ONTD VWD GA Incorrect Perinatal  Complications*
	Within Normative Range		Low Risk	ONTD VWD Perinatal  Complications*
	High	Down Syndrome Chromosomal Abnormalities GA Incorrect Perinatal  Complications*		ONTD VWD Twins Perinatal  Complications*

REFERENCES

1. Carter PE, Pearn JH, Bell J et.al. Survival in trisomy 18. Life tables for use in genetic counseling and clinical pediatrics. Clin Genet 1985;27:59-61.
2. Moerman P, Fryns JP, Goddeeris P et.al. Spectrum of clinical and autopsy findings in trisomy 18 syndrome. J Genet Hum 1982;30:17-38.
3. Taylor AI. Autosomal trisomy syndromes: a detailed study of 27 cases of Edwards' syndrome and 27 cases of Patau's syndrome. J Med Genet 1968;5:227-252.
4. Hill LM. The sonographic detection of trisomies 13, 18 and 21. Clin Obstet Gynecol 1996;39(4):831-850.