Double marker alone is not all. Do combine with maternal age, NT,NB, FHR also in MoM / Software for calculation of probability

 

ABC of double marker: In normal Euploid pregnancies

b-bCG is 1.0 MoM

Double marker is  :-serum biochemistry: --

Point1: The gestational age used for the calculation of biochemical risk must be derived from the CRL and is calculated automatically by the various software’s available today on the basis of the CRL at the time of the 11-13+6 week’s scan.

 Point 2 what to tests in serum??  : Maternal serum frees b-hCG and PAPP-A are the two main chemical markers evaluating the fetal aneuploidy today.

Point 3 : There is no significant association between fetal NT and maternal serum free b-hCG or PAAP-A in either Trisomy 21 or chromosomally normal pregnancies.

Point 4:- So in a given case the ultrasononographic and biochemical markers shold be combined to provide more effective screening than either method individually. If we insist on both the test in a woman then the detection rate for Trisomy 21 will be  86%   at a 5% false positive rate.

Point  5:-If we combine a) maternal age 2) NT 3) FHR   4) Free beta HCG   & 5)  PAPP-A  then about 90% of Trisomy 21 pregnancies will be picked up for a false positive rate of 3%.  Point 6: Why maternal age & NT report were included in the software for risk assessment??

Ans: For a health foetus PAPP-A is 1.0 MoM screening. But if we combine maternal age and fetal NT thickness & PAPP-A in a software then (by adding 2  more factors i e. Age & NT report more parameters) at 11-13+6 weeks of gestation then such software will  identify about 95% of affected fetuses for a screen-positive rate of about 5%. Subsequently, maternal age was combined with fetal NT and maternal serum biochemistry (free b-hCG and PAPP-A) in the first-trimester to identify about 90% of affected fetuses with false positive rate of 3%.

Point 7: Should we include many more parameters in Trisomy in the calculation of probality risk  software / estimation of MoM?? Ans:  Introduction of other ultrasonography markers like fetal nasal bone improves the detection of Trisomy 21 up to 95%.Method of screening, A) Maternal age (MA).B ) MA and fetal nuchal translucency (NT) at 11-13.6 weeks C) MA and fetal NT and fetal nasal bone (NB) at 11-13+6 weeks D) ) MA and fetal NT and maternal serum free b-hCG and PAPP-A at 11-13+6 weeks      E) MA and fetal NT and NB and maternal serum free b-hCG and PAPP-A at 11-13+6 weeks  F) MA and maternal serum biochemistry at 15-18 weeks .From above discussions it is evident that inclusion of  as many as factors possible along with chemical markers in maternal evaluation plays important role in improving the detection rate of Trisomy 21.