OB/GYN
Prenatal Aneuploidy Screening
OB/GYN

Prenatal Aneuploidy Screening

First/second trimester screens and diagnostic testing.

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◆Diagnostic testing

  • •CVS: 10–13 weeks (chorionic villus sampling)
  • •Amniocentesis: ≥15 weeks
  • •NEVER skip directly to termination counseling without confirmatory testing

◆AFP interpretation

  • •↑ AFP causes:
  • • Open neural tube defects (anencephaly, spina bifida)
  • • Abdominal wall defects (gastroschisis, omphalocele)
  • • Multiple gestation
  • • Incorrect dating (most common cause)
  • •↑ AFP is NOT a sign of T21 (T21 has LOW AFP)
  • •MSAFP + acetylcholinesterase in amniotic fluid → NTD confirmation

◆Anatomy ultrasound

  • •18–20 weeks
  • •Major structural anomalies, NTD, cardiac, renal, abdominal wall, limbs

Screening patterns

TestTimingComponentsT21 patternT18 pattern
First trimester10–13 weeksβ-hCG + PAPP-A + nuchal translucency (NT)↑ hCG, ↓ PAPP-A, ↑ NT↓ hCG, ↓ PAPP-A, ↑ NT
Quad screen15–20 weeksAFP + β-hCG + estriol + inhibin A↑ hCG, ↑ inhibin, ↓ AFP, ↓ estriolAll LOW (and ↓/normal inhibin)
Cell-free fetal DNA (NIPT)≥10 weeksFetal DNA in maternal bloodPositive screenPositive screen

High-yield pearls

  • ◆T21 quad: ↑hCG + ↑inhibin + ↓AFP + ↓estriol (only hCG/inhibin up)
  • ◆T18 quad: all LOW
  • ◆↑ AFP = open defect (NTD or abdominal wall), NOT T21
  • ◆Cell-free DNA is highly accurate but still a SCREEN — confirm with CVS or amnio
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