Our genetic material, or DNA, is organised into 46 packages called chromosomes. Some people are born with too many or not enough chromosomes, which can affect their health and learning. Smaller changes in the DNA can also occur within a chromosome, resulting in a variety of genetic conditions. Every baby has a small chance of being born with a chromosomal or genetic condition. Testing during pregnancy for some of these conditions is called prenatal screening. This is available to provide you with more information about your unborn baby. It is your choice whether to have any prenatal screening tests. You should only have testing if you understand what the test is for and what the results are able to tell you.
Screening tests
The principle of screening is to offer a safe, accessible test to identify women with an increased chance of having a baby affected by a chromosomal or genetic condition. These women with an increased chance are offered genetic counselling and follow up diagnostic testing (see below under ‘Diagnostic tests’). It is very important to understand that a screening test does not tell you for certain if your baby has the condition, only if there is an increased chance. Screening tests may also miss some babies that have the condition. Your doctor or midwife should explain the results of your screening test to you and refer you to a genetic counsellor if required.
Which chromosomal conditions can be screened for?
The most common chromosomal cause of intellectual disability in children and adults is Down syndrome. This condition occurs when a baby has three copies of chromosome 21 (trisomy 21), instead of the usual two copies. The chance of a woman having a baby with Down syndrome increases with her age. Because of its frequency in the population (about 1 in 800) and its effects on health and learning, prenatal screening for Down Syndrome is made available to all Australian women. Other chromosomal conditions that may be screened for with the test are Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13). These are serious conditions associated with pregnancy loss, disability or death of the newborn.
Types of prenatal screening tests for Down syndrome and other chromosomal conditions
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