In each cell in our body there are typically 46 chromosomes, with the DNA in our chromosomes determining how we develop. Down syndrome is a genetic condition caused when there is an extra chromosome, meaning people with Down syndrome have 47 chromosomes in each cell instead of 46. It is the most common chromosome disorder occurring at contraception, affecting over 270 babies born in Australia per year since 2007 to parents of all ethnic, social and age groups. The population of people with Down syndrome in Australia is now over 13,000.
Down syndrome is caused by a random error in cell division, leading to an extra chromosome copy of chromosome 21. When a cell divides into two, usually one pair of chromosomes goes into one cell and the other pair goes into the other cell. in Down syndrome, nondisjunction error occurs where both pairs of chromosomes go into one cell and no chromosomes go into the other cell. The error commonly and randomly occurs during the formation of an egg or sperm with no known cause, leading to all the person’s cells having three copies of chromosome 21 instead of two. This is known as complete trisomy 21 and is the cause of 95% of Down syndrome cases. At this stage in time, no environmental or behavioural factors of parents is known to cause Down Syndrome however in more than 90% of cases the extra chromosome copy of chromosome 21 comes from the mother. However, advancing maternal age increases risk of giving birth to a child with Down syndrome as older eggs have a greater risk of error in chromosome division.
Down syndrome is commonly recognised at birth and diagnosis is confirmed through a blood test but tests can be carried before the baby is born. After birth, Down syndrome is often suspected based on the baby’s appearance and a blood test called a chromosomal karyotype will be conducted. This test will analyse the child’s chromosomes and if an extra 21 chromosome is present in some or all cells then a diagnosis of Down syndrome is confirmed.
Screening tests for Down syndrome is also offered to all pregnant women in Australia and while it is not a definite diagnosis it can tell you how likely the baby has Down syndrome. In the first trimester, screening involves a blood test and an ultrasound scan and is offered while the woman is between weeks 9-13 weeks 6 days of pregnancy. The blood test measures hormone levels in the blood which change during pregnancy free B-hcG and PAPP-A, with a presence of abnormal hormone levels indicating there may be a genetic condition. During the
ultrasound, the thickness of the nuchal translucency, a pocket of fluid at the back of the baby’s neck is measured with the nuchal translucency being larger in babies with Down Syndrome. Using the blood test and ultrasound, the chance of the baby having Down syndrome is worked out.
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