http://indiagynaecology.blogspot.com/pregancny/my-post-on-pregnancy)

The commonest cause of mental retardation in children is downs Syndrome.1 in 700 women runs the risk of having a Downs baby.The associated risk factors are increasing maternal age,a family history of Downs syndrome,a previous child with Downs syndrome.Having said that it doesn't mean that a Lady of 40 will for sure have a Downs baby and Lady of 20 won't have one.So now WHO recommends 'screening tests,'(a test which tells the risk but doesn't confirm the condition).A blood test double marker which can be between 11 to 13 weeks or triple marker between 16 to 18 weeks.One doesn't need both the tests.The other mandatory test is level 1 Ultrasound Scan,which measures the thickness of baby's nape of neck and nasal bone thickness.If unfortunately one is high risk in the screening test one has the option to go for confirmatory CVS and amniocentesis.