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 scans (or NIPT) suggest a higher risk. Amniocentesis and CVS 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 an injection of local anesthetic under the skin and then taking a small sample from the placenta, using a needle under continuous ultrasound vision. You will be able to go home 30 minutes after the test. The risk of this causing a miscarriage is under 1 in 100. Limb abnormalities have rarely been observed in babies who underwent CVS before 10 weeks, so we never offer CVS before 11 weeks' gestation. A rapid analysis 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, using a very thin needle under continuous ultrasound vision. Local anesthetic is not necessary. You will be able to go home 30 minutes after the test. The risk of miscarriage is similar to a CVS i.e. under 1 in 100. A rapid analysis result using the same techniques (PCR and FISH) can usually be obtained within 2 working days.