Kar the chemistry for downs is blood work.
Now NT ultrasound is done in conjuction with bloods. This gives a more accurate picture of a baby's development. I know things can be a little different when done through a public hospital. It takes time for the results to come back to the mother and I believe not all hospitals do the blood work in conjunction with the u/s.

If you are having nt u/s with a private ob you will sometimes be sent to a foetal medicine specialist - usually an ultrasonographer will do the initial scanning and measurements and then the foetal med. ob will come in and check it out. You will then be given your risk assessment on the spot it will be given as a 1:??? result. You will be told what your risk factor is just based on age 1:300 for example and then be given your result when the nuchal fold is measured combined with the risk factors from the blood work.

Most specialist ob's prefer to do the nuchal fold at around 12.5 weeks - this gives the best view for measurement of the nuchal fold. After about 13.5 weeks the nuchal fold begins to disappear as the baby grows so it is more difficult to measure. Most specialist ob's will require that you have the blood work done a few days prior to the u/s so they have the results to combine with the measurements.

The combination of blood work and u/s measurement of the nuchal fold and u/s examination of the baby for other soft markers is thought to give the accuracy of nt u/s up around 85% - 90%. Without blood work the accuracy is decreased. This is because some babies with chromosomal defects do have a normal nuchal fold measurement. The extra info that the blood work gives gives a more rounded picture.

NT U/s is not a definite diagnosis and when there are any red flags you will be given the option of amniocentesis or cvs. I believe it's really important to ask your ob what the accuracy rate of the practitioner you go to see has. For instance I worked with a foetal med. ob and his nt u/s accuracy was 100 per cent. Meaning that he had never had a case slip through.

Some families are more comfortable with waiting and hoping that there little one is fine and embrace whatever comes along. For other families they need to know with reasonable certainty that their little ones are 'okay'. Niether is right or wrong it is a very personal and emotional choice. It is erroneously believed that chromosomal abnormalities of which Down's Syndrome is the most common is confined to 'older' women. This is patently incorrect. Yes, the risk factors increase with age but it is now believed that there are links with nutritional deficiencies and down's syndrome as well (that's a whole other story!). I have a friend who had her son who was born with Down's Syndrome (Trisomy 21) when she was 27. She went on to have 3 other children her last at 40. Her other children were not afflicted with Trisomy 21. She has done mountains of research and there appears to be a very strong link with low levels of folate and zinc and higher incidences of Trisomy 21....

Anyway if it is important for a family to feel reasonably secure that there baby is not afflicted with a chromosomal abnormality I think NT U/s is a very good option. However it is my opinion that it is important to have it done with a foetal med ob.