About 75% of babies with Down's syndrome (Trisomy 21) will have an increased Nuchal translucency (this is an increased amount of fluid at the back of the babies neck). Also the nasal bone will not be present in 60-70% of Down's fetuses.  These features can be seen on ultrasound (nuchal scan) between 11-14 weeks of gestation.

By means of a blood test at the same gestation, the maternal serum concentration of free βhCG (a hormone) and PAPP-A (a protein) can be analysed. It has been found that the level of βhCG is higher in Down's fetuses than in normal fetuses whereas the level of PAPP-A is lower than in normal fetuses. 

• Screening by maternal age alone will pick up only 30% of babies with Down's syndrome.
• Screening by combining maternal age and a blood test at 15-18 weeks will pick up 50 -70%.
• Screening by combining maternal age with nuchal scan at 11- 14 weeks the pick up rate is 70- 80%
• Screening by combining maternal age,  nuchal scan and blood biochemistry (for maternal serum free β hCG and PAPP-A ) will give a pick up rate of 90%.

i.e. this combined testing will correctly identify 90% of babies with Downs' syndrome without the need for invasive testing.

The results of the tests are expressed as a probability of the baby having Down's syndrome; this is expressed as a ratio e.g. 1:500 means that out of 500 women one will have a baby with DS.

Low risk is considered as a result where the risk is less than one in 300. This means that out of 300 ladies with this risk factor one will have a baby with Down's. High risk is considered when the risk is more than 1 in 300.

Therefore, NT screening is not a completely accurate diagnostic test and so cannot give a definite yes or no result. Occasionally a baby will be born with DS when the OSCAR result has shown a low risk. Conversely such screening tests can occasionally also give you a false positive result which means it incorrectly identifies a low risk pregnancy as a high risk. The only way to know for sure whether the fetus has a chromosomal abnormality is by having an invasive test such as an amniocentesis or Chorionic Villus sampling (CVS).

However by opting for nuchal scan and blood testing you will be given a risk factor, from which you can make the best informed choice as to whether to proceed with invasive testing such as amniocentesis or Chorionic Villus Sampling.

Definitive Diagnosis of Down's Syndrome
The only way to know for sure whether or not your baby has a chromosomal abnormality is by having an invasive test such as a CVS or amniocentesis. These are called 'diagnostic tests' and give a definite answer. However, these tests carry a risk of miscarriage.

Chorionic Villus Sampling (CVS)
A sample of placenta is taken using a fine needle through your tummy. The placenta is localised using an ultrasound scan. CVS can be carried out early in pregnancy; from the 11th week after the last period. There is a quoted risk of miscarriage by this procedure of about 1.5 - 2.5%

Amniocentesis (Sampling amniotic fluid)
Again using ultrasound to guide a needle through your tummy this involves withdrawing fluid from around the fetus. Cells in the fluid are examined for the chromosome count. This procedure can be carried out from the 14th week after the last period. The procedure does carry a risk of miscarriage quoted at about 0.5 - 2%