###Human: In NT scan,Trisomy 21 screen is positive. What does it mean? ###Assistant: Hello. 1 in 250 means that if we take 250 women of the same results as your wife had then only one will have a baby with Down syndrome. So this is the explanation of the calculated risk of down syndrome. Secondly, NT (nuchal translucency) and blood tests which she underwent are screening tests. Screening tests are not diagnostic. They only detect who is at risk. If they become positive, that does not mean that particular is at risk. It just says that specific condition may or may not be there which is then confirmed by diagnostic tests. In your case, CVS (chorionic villus sampling, taking a sample from the baby) will diagnose if the Down syndrome is there or not which you will be offered at 12 to 13 weeks most probably. I hope it has helped you. If you require any further queries, please do not hesitate to contact me.