What is Down Syndrome?
People with Down syndrome have an extra chromosome
having three copies of chromosome 21 instead of two. (Down
syndrome is also known as trisomy 21 for this reason).
Chromosomes carry genes that pass characteristics from parents to their children. Abnormalities can occur when there are too many chromosomes. Children with Down syndrome will have learning disability and may have other abnormalities such as heart defects.
Older mothers are more likely to have a baby with Down syndrome. For example, Down syndrome occurs once in every 1500 births to women aged 20 years whereas at the age of 40 the rate is increased to 1 in 100 births.
It is now possible to effectively identify pregnancies at increased risk of Down syndrome. This involves combining the results from an ultrasound scan (nuchal translucency scan) with results of hormone levels in maternal blood (serum test). The figure opposite shows a nuchal scan.