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ONE TOO MANY

© Terrie-Lynn Daley

Down syndrome is a genetic condition where there are three 21st chromosomes instead of two. Most people have 46 chromosomes per cell, 23 chromosomes in the mother's egg and 23 in the father's sperm.Children with Down syndrome, also referred to as Trisomy 21, have 47 chromosomes per cell, which includes the extra chromosome #21. There is also a rare type of Down syndrome where some of the cells have 46 chromosomes, while others have 47. This is known as Mosaicism, because the chromosomes take on a Mosaic type pattern.

More than 50 percent of miscarriages are caused by a chromosomal abnormality. As many as 25 percent of all miscarriages are caused by a \223trisomy,\224 which is the presence of three copies of a particular chromosome, rather than the normal two. 20 to 25 % of children conceived with Down syndrome survive past birth.

Down syndrome is named after Dr. John Langdon Down, an English physician who first described the characteristics of Down Syndrome or Trisomy-21. It affects all ages, races, economic levels, and is one of the most frequently occurring chromosomal abnormalities, occurring in 1 out of every 800 to 1,000 children born.

Down Syndrome results when an error in cell division, called non-disjunction occurs. There are two milder types of chromosomal abnormalities, referred to as Mosaicism and Translocation, which are also implicated in Down syndrome. Regardless of the type, children with Down syndrome have an extra portion of the number 21 chromosome present in all, or some, of their cells. This additional genetic material changes the development of a child and causes characteristics associated with the disorder. Mosaicism is rare, responsible for only 1 to 2 percent of all cases of Down syndrome. Translocation occurs in only 3 to 4 % of people with Down syndrome. Approximately in one-third of cases, one parent is a carrier of a translocated chromosome.

80 % of children born with Down syndrome are born to women under 35 years of age, due to the fact that younger women have higher fertility rates. The incidence of children with Down syndrome increases with the age of the mother, however. There seems to be no connection between Down syndrome and parents' activities, before or during pregnancy.The risk of having a second child with Trisomy 21 or Mosaic Down syndrome is about 1 in 100. The risk is higher if one parent is a carrier of a translocated cell.

There are two types of procedures available to pregnant women to determine the chance of having a Down Syndrome child and if the child actually has the disorder. Screening Tests estimate the risk of the fetus having Down Syndrome, and Diagnostic Tests determine whether or not the fetus actually has the condition. The most commonly used screening tests are the Triple Screen and the Alpha-fetoprotein Plus. These tests measure quantities of various substances in the blood, including alpha-fetoprotein, human chorionic gonadotropin and unconjugated estriol, and together with the woman's age, estimate her risk of having a child with Down

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