Down Syndrome Down syndrome is one of the most common and easily recognized genetic disorders. It is caused by abnormal cell division that involves the 21st chromosome pair of the 23 pairs of chromosome found in all normal human cells. There are three types of these abnormalities, which are non-disjunction, translocation, and mosaicism. Ninety five percent of children born with Down syndrome are a non-disjunction type which is three rather than two chromosome 21s. About three to four percent of cases are because of translocation which is chromosome 21 detaches and attaches to another chromosome. Mosaicism accounts for only one to two percent of births, which is an abnormal cell division with the normal 46 chromosomes while the others have 47 creating a mosaic of normal and abnormal cells (Encarta.p1). The presence of Down syndrome in children is also due to the age of the mother. A twenty five year old woman' s chance of having a baby with Down syndrome is 1 in 1,250, but a woman who is thirty five it's 1 in 378 and a woman 45 it's1 in 30 (Encarta.p2). Diana Danilenko-Dixon, M.D. states "As a woman's eggs age, there is a greater propensity for chromosomes to divide improperly". Although researchers are studying ways to detect improper cell division, there isn't a test yet to the likelihood of Down syndrome before conception (Exceptional children p173). One recent study noted that a rise in children born with Down syndrome between 1990 and 1995 because of the old maternal age. The age of the father does not seem to be a factor. When a woman becomes pregnant, there are screening tests that can be performed to detect Down syndrome beginning at the tenth week of gestation. Of course the test isn't 100% accurate so you should know a positive result does not mean Down syndrome is present and a normal screening doesn't mean the baby will be born without Down syndrome; the chance is just reduced.
