With every pregnancy, there is a small chance of having a child with Down's syndrome.
There's currently no clear evidence to suggest that anything done before or during pregnancy increases or decreases this risk, although some people are more likely to have a child with Down's syndrome than others.
The main factor that increases the chance of having a baby with Down's syndrome is the age of the woman when she becomes pregnant. For example, a woman who is:
20 years of age has a risk of one in 1,500
30 years of age has a risk of one in 800
35 years of age has a risk of one in 270
40 years of age has a risk of one in 100
45 years of age has a risk of one in 50 or greater
The chance of having a child with Down's syndrome is also increased if you have previously had an affected pregnancy.
source :- http://www.nhs.uk/Conditions/Downs-syndrome/Pages/Causes.aspx
