These are diagnostic tests that will give a yes or no answer, as to whether the baby has Down's Syndrome. However, they carry a small risk of miscarriage and are therefore only usually performed when the combined test suggests a higher risk. These tests are available on the NHS, but usually only if the risk is worse than 1 in 150.

Chorionic villus sampling (CVS)

This can be performed from 11 weeks. It involves taking a small sample from the placenta, under continuous ultrasound vision. The risk of this causing a miscarriage is about 1 in 100-200. Limb abnormalities have been suggested in babies who underwent CVS before 10 weeks, so we never offer CVS before 11 weeks' gestation. If you have this test performed privately by Mr Impey, a rapid result using special techniques (PCR and FISH) can usually be obtained within 2 working days.

Amniocentesis

This is done from 15 weeks. It involves taking a small amount of amniotic fluid from around the baby, under continuous ultrasound vision. The risk of miscarriage is about 1 in 200. If you have this test performed privately by Mr Impey, a rapid result using special techniques (PCR and FISH) can usually be obtained within 2 working days.

Amniocentesis has a slightly lower miscarriage rate than CVS but can only be performed from 15 weeks.