What is the normal range of Nuchal translucency (NT) scan test?

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For those who may not be sure, the NT scan is for assessing your baby's risk of having Down syndrome (DS) and some other chromosomal abnormalities as well as major congenital heart problems. You'll get your results in the form of a ratio that expresses your baby's chances of having a chromosomal defect. For example, if your result is 1 in 30 or 1 in 4,000. This would mean that your baby has a risk of 1 in 30 which is for every 30 women with this same result, one baby will have Down syndrome and 29 will not. A risk of 1 in 4,000 means that for every 4,000 women with this result, one baby will have it and 3,999 will not. The higher the second number, the lower the risk. If you are told that your results are "normal" or "abnormal," which will depend on whether they are below or above a specified cutoff. For example, some tests use a cutoff of 1 in 250. So a result of 1 in 4,000 would be considered normal because the risk that there's a problem is lower than 1 in 250. A ratio of 1 in 30 would be considered abnormal because that risk is higher than 1 in 250.

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Less than 3mm ideally but sometimes babies with upto 3.5mm can be normal without any abnormality. Consult your gynaecologist definitely.