What is Trisomy 18
Trisomy 18 is a genetic disorder caused by a chromosomel defect. This is not due to inherent defect in any chromosome, rather the affected individuals have an additional copy of chromosome 18. Trisomy 18 is also called Edwards syndrome, named after John H. Edwards, who first described the syndrome in 1960.
In humans each cell normally contains 23 pairs of chromosomes, 23 obtained from the father and 23 from the mother. Chromosomes are identified by numbers, for example in the humans they are 1 through 23. The 23rd chromosome being the sex chromosome, X or Y chromosome, determining the sex of the individual. Trisomy 18 occurs when the individual has an additional chromosome 18 resulting in a total of 47 chromosomes.
Trisomy 21, or Down syndrome, is the most common trisomy occuring at a frequency of 1 in 800 births. Trisomy 18 is the second most common trisomy (1 in 6000 births), followed by Trisomy 13 or Patau syndrome (1 in 10,000 births). Trisomy 21 is due to an extra chromosome 21 while Trisomy 13 has three chromosome 13. For the condition to manifest, it is not necessary to have the complete copy of the extra chromosome; a part of the extra chromosome is sufficient to cause the abnormality.
According to Trisomy 18 Foundation, "unlike Down syndrome, which also is caused by a chromosomal defect, the developmental issues caused by Trisomy 18 are associated with medical complications that are more potentially life-threatening in the early months and years of life. 50% of babies who are carried to term will be stillborn, with baby boys having higher stillbirth rate than baby girls." They add, "a small number of adults (usually girls) with Trisomy 18 have and are living into their twenties and thirties, although with significant developmental delays that do not allow them to live independantly without assisted caregiving."
Isabelle, or "Bella" Santorum, daughter of Rick Santorum, a 2012 U.S. Presidential candidate, was born with Trisomy 18.
When there is an abnormal number of chromosomes due to an extra or missing chromosome, the condition is generally called aneuploidy. Normally every cell in our body except the gametes contains diploid number of chromosomes (46 or 23 pairs). The gametes will have only haploid number (23).
Prenatal screening may provide important information on chromosomal anomaly in the fetus. Though there is an inherent risk associated with invasive techniques, a positive identification of aneuploidy may allow the couple to choose to continue the pregnancy or not. It has been estimated that 6 in 1,000 births may carry chromosomal abnormalities. The incidence of aneuploidy has been known to increase with advancing maternal age.
Aneuploidy screening in the first trimester Kevin Spencer. American Journal of Medical Genetics Part C: Seminars in Medical Genetics. Volume 145C, Issue 1, pages 18–32, 15 February 2007.
Is there a paternal age effect for aneuploidy? Fonseka KG, Griffin DK. Cytogenet Genome Res. 2011;133(2-4):280-91. Epub 2011 Jan 6.
The first-trimester screen in clinical practice. Knutsen-Larson S, Flanagan JD, Van Eerden P, Stein QP. S D Med. 2009 Oct;62(10):389, 392-3.
Malignancy in children with trisomy 21. Rabin KR, Whitlock JA. Oncologist. 2009 Feb;14(2):164-73. Epub 2009 Jan 28.
Trisomy 18 Foundation. The Trisomy 18 Foundation was started by Don and Victoria Miller after the loss of their son Isaac in 2001. The mission is to encourage the search for treatments and preventions of Trisomy 18, to educate and support medical professionals, and to create a caring worldwide community for affected families.
Pubmed Health. A U.S. National Library of Medicine resource. It says "Trisomy 18 is a relatively common syndrome. It is three times more common in girls than boys. The syndrome is caused by the presence of extra material from chromosome 18. The extra material interferes with normal development."
What is trisomy 18? NIH Genetics Reference. A guide to understanding genetic conditions. This is also a resource provided by the U.S. National Library of Medicine. It says Trisomy 18 "affected individuals may have heart defects and abnormalities of other organs that develop before birth. Other features of trisomy 18 include a small, abnormally shaped head; a small jaw and mouth; and clenched fists with overlapping fingers. Due to the presence of several life-threatening medical problems, many individuals with trisomy 18 die before birth or within their first month. Five to 10 percent of children with this condition live past their first year, and these children often have severe intellectual disability."
Trisomy (Wikipedia). A description of trisomy in general.
Trisomy 18 Foundation Support site. A trisomy 18 support group.