Brief Answer:
EXPLAINED BELOW
Detailed Answer:
Hello
I understand your apprehension.
At no point did I mean that you were a statistic, its just that unfortunately , the reports that we have deal with statistics, and it is important to understand them.
Let me elaborate.
The CRL , NT and blood values together only SCREEN you for Downs.
There is an accepted cut off level, say 1 : 150.
If a lady has a risk higher than this ,
amniocentesis is recommended.
If it is lower than this ( it could be 1 : 250 or 1 : 1000 ) , then it is accepted that the risks of amniocentesis outweigh the benefits of knowing the results.
That is why further testing is not recommended for you.
Now for your questions one by one -
1. Normally, NT values above 2.5 mm are considered high risk for Downs, but it is futile to comment on an isolated NT measurement, as the screening test is COMPREHENSIVE, not based on a single value.
2. Regarding the biochemical markers, Maternal serum markers (in this case the free beta-
hCG and the PAPP-A) steadily change as the
pregnancy progresses (i.e., by weeks' gestation) and these changes are quite significant during this early part of the pregnancy while the placental tissues are rapidly proliferating. During pregnancies in which the baby is chromosomally normal, the hCG levels begin to drop toward the end of first trimester and then level off and the PAPP-A levels tend to increase gradually throughout the pregnancy. In the case of a baby with Down syndrome (
trisomy 21), the hCG levels are often elevated and the difference between chromosomally normal babies and those with Down syndrome increase as the pregnancy progresses.
Because of the changes in serum marker levels by
gestational age, for accurate interpretation of the test results, a different reference range must be used for each week of gestation, depending on when the test is drawn. To avoid the multiple reference range problems and also to standardize test results between different laboratories, whose reference ranges also vary, a MEDIAN value for test results in normal pregnancies is determined in each laboratory for each week of gestation.
Then the values are expressed as multiples of that ' normal ' median. You have mentioned absolute results, not the MoM values, I would need those, and even then, it is to a point futile, because again, only the WHOLE combined risk matters, not individual markers.
3. A second nuchal scan and blood screen would be advised if you do not have faith in the first result, due to whatsoever reasons.
4. There is a long list of chromosomal disorders that could result due to
advanced maternal age, most are rare, the commoner ones are Downs syndrome, Trisomy 13 and Trisomy 18. Amniocentesis tests for the latter three, not all of them.
5. There is no other test to consider, there is no test available in the world that can test for ALL possible chromosomal anomalies.
6. If the amniocentesis is positive, then chances are negligible that the baby wont have Downs. It is a very specific test in that sense, unlike the screening tests, which are not specific.
7. IF the amnio is negative, again, the chances of the baby having Downs are very low, since the test is quite specific.
8. Amniocentesis cannot be inconclusive for Downs, it will tell you either ways.
At best, the NT and blood values give you an ESTIMATE of your baby having Downs.
Based on the estimate, further testing is recommended. YOu do not come under that category, where further testing is warranted, however, it is an individual choice.
If you want 100 % assurance and NOT an estimate, you have to choose .
Hope this is a bit more helpful.