All about diagnostic tests
If a pregnancy is considered high risk (where screening results show possible abnormalities, the mother is over 35 or one parent has a family history of genetic disorder), you may consider chorionic villus sampling (CVS) and/or an amniocentesis.
Both tests detect Down syndrome, chromosomal abnormalities and certain genetic disorders.
CVS is performed between weeks 11 and 14, either vaginally, similar to a Pap smear, or abdominally, where a needle is passed into the placenta through the belly. A sample of placental tissue is removed for analysis. The risk of miscarriage associated with CVS is inconclusive but thought to be about one in 100.
An amniocentesis is carried out between weeks 15 and 18. A needle is passed into the stomach and amniotic fluid is withdrawn.
The risk of miscarriage is less than one per cent.
A fairly new test, non-invasive prenatal testing (NIPT) can be helpful for women at moderate to high risk for chromosomal abnormalities. This involves isolating foetal DNA fragments from a blood sample.