Nuchal translucency (NT) is the sonographic appearance of a subcutaneous collection of fluid behind the fetal neck. The optimal gestational age
for the measurement of fetal NT is between 11 weeks and 13 weeks 6 days.
At 12 weeks of gestational age, an "average" nuchal thickness of 2.18mm has been observed, however, up to 13% of chromosomally normal fetuses present with a nuchal lucency of greater than 2.5mm, and thus for even greater accuracy of predicting risks, the outcome of the nuchal scan should be combined with the results of simultaneous maternal blood tests. ( B- HCG and PAPP-A ). Other risk factors such as maternal age, diabetes
, previous history of abnormal babies etc . are also considered.
Nuchal scanning alone detects 62% of all Down's Syndrome with a false positive rate of 5.0%, the combination with blood testing gives corresponding values of 73% and 4.7%. This means that in 5 % of cases, the result is positive, wherein no abnormality exists. Finally, using the additional ultrasound
feature of an absent nasal bone
can further increase detection rates for Down syndrome to more than 95%.
When screening is positive, Chorionic villus sampling
(CVS) or amniocentesis
testing is required to confirm the presence of a genetic abnormality. However this procedure carries a small risk of miscarriage
so prior screening with low false positive rates are needed to minimize the chance of miscarrying.
If the CVS is normal, then further on , ultrasound scans will test the baby for any abnormalities not picked up by CVS, such as a structural defect in the heart etc.
At the most, what these tests offer you is a reasonable surety that your baby will be born normal.
There is no test that can tell you that 100 % you will have a normal baby.
It is a calculated risk that you will have to take alongwith your partner.
I suggest you go ahead for CVS to assure yourself, but please consult a good genetic counsellor first who will explain all the details to you.
All the best for you and your baby .