Q. In NT scan,Trisomy 21 screen is positive. What does it mean?

Answered by
Dr. Nadia Khan
and medically reviewed by Dr.Nithila A
This is a premium question & answer published on Apr 12, 2019

Hello doctor,

My wife is 36-year-old with a height of 5.6 and weight 69.1 kg. She had NT or NB scan and double marker test in obstetrical ultrasound. The nuchal thickness is 1.0 mm. Presence of nasal bone. DV flow is normal, but in double marker test, free beta hCG is 46.20 and PAPP-A is 2.58. Trisomy 21 -screen positive cut off 1: 250. Mother's cut off free beta hCG is greater than or equal to 1.98 PAPP-A.



Welcome to

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.

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