Aneuploidy screening should be offered to all pts. Counseling includes what is being screened for, potential results, advantages/disadvantages (including cost), & how the results might impact their decisions about the Preg.
Reported as “risk” of aneuploidy (w/ regard to trisomy 21 & trisomies 13/18) compared to age-matched reference, not as positive or negative (except for cell- free fetal DNA, see below). Overall: 5% positive screen rate (predetermined).
Screening parameters: Combination of values used in various screening approaches
NT: Defined anatomic area behind fetal neck measured sonographically as width (mm) btw ∼11–14 w. ↑ in aneuploidy & other conditions. Lower false positive rate if combined w/ serum markers. Useful in multiples when serum markers not accurate (ie, each fetus evaluated).
NB: Used w/ NT for trisomy 21 eval
Serum markers: Preg hormones used in combination to calculate risk (AFP, β-hCG, PAPP-A, inhibin A, UE3)
1st trimester screening: NT, PAPP-A, & β-hCG in mat serum at 11–14w.
Comparable detection rates to 2nd trimester screen but higher screen positive rate in women >35 yo compared to 2nd trimester screen.
Advantages: Time for CVS as diagnostic test & earlier termination options.
Disadvantages: More costly approach. In case of sequential strategy, pts must wait for results until 2nd trimester.
2nd trimester screening: AFP, hCG, unconjugated estriol (UE3),and inhibin A in screen at 15–18 w & some labs up to 24+ weeks. Detection 69% for triple screen, 81% for quadruple screen (using inhibin A).
Advantages: Does not rely on NT (operator dependent test). Serum markers may suggest other problems (eg, ↑ AFP for NTD).
Disadvantages: Only screening → amniocentesis for dx. Given later GA, if anomaly found, options may be more limited.
Combined approaches: Uses both 1st & 2nd trimester screening protocols.When 1st & 2nd trimester protocols used independently, false positive rate ↑.
Integrated screening: Integrates 1st & 2nd trimesters → results given in 2nd trimester 94–96% detection rate w/ full integrated (NT, PAPP-A, quad screen)
Sequential screening: 1st & 2nd trimester screens performed w/ results reported after 1st & then altered after 2nd trimester.
Benefits:Allows CVS for those at highest risk & ↓ anxiety of waiting. 95% detection rate by 2nd trimester.