Prenatal screening

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Prenatal Screening - A Trimester Tour

A sequential process to screen for aneuploidy and neural tube defects. Non-invasive options (cfDNA from 10 wks) have high sensitivity but are not diagnostic. Definitive diagnosis requires invasive testing.

Fetal Nuchal Translucency Ultrasound Measurement

Down Syndrome (Trisomy 21) Quad Screen: shows ↑ β-hCG, ↑ Inhibin A, ↓ AFP, and ↓ unconjugated estriol (uE3). 📌 Mnemonic: HIgh levels (HCG, Inhibin A).

Aneuploidy Analytes - Biomarker Breakdown

  • First Trimester Screen (Weeks 9-13): PAPP-A + β-hCG + Nuchal Translucency
  • Second Trimester Quad Screen (Weeks 15-22): AFP + β-hCG + Estriol + Inhibin A
AnalyteTrisomy 21 (Down)Trisomy 18 (Edwards)Neural Tube Defects
AFP
β-hCGNormal
EstriolNormal
Inhibin ANormal or ↓Normal
PAPP-AN/A

High Yield: Inhibin A is the most specific second-trimester marker for Down syndrome, as it is typically normal in other trisomies.

cfDNA vs. Diagnostic Tests - Screening vs. Certainty

  • cell-free DNA (cfDNA)

    • Screening test using maternal blood; non-invasive.
    • High detection rate for common aneuploidies (T21, T18, T13).
    • Performed at or after 10 weeks gestation.
    • ⚠️ A positive cfDNA result always requires confirmation.
  • Invasive Diagnostic Tests

    • Provide certainty via fetal karyotype.
    • Carry a small risk of procedure-related fetal loss.
    • Chorionic Villus Sampling (CVS): 10-13 weeks.
    • Amniocentesis: >15 weeks.

⭐ cfDNA is a powerful screening tool, but only invasive tests like amniocentesis or CVS provide a definitive diagnosis. False positives on cfDNA can occur, making confirmatory testing essential.

NTDs & Abdominal Wall - Finding the Flaws

  • Initial screen: ↑ maternal serum AFP.
  • Neural Tube Defects (NTDs):
    • Anencephaly: "Frog-like" facies, polyhydramnios.
    • Spina Bifida: Look for "lemon" (frontal bones) & "banana" (cerebellum) signs on ultrasound.
  • Abdominal Wall Defects:
    • Gastroschisis: Right of midline, no sac.
    • Omphalocele: Midline, sac present.

Lemon Sign in Fetal Ultrasound

⭐ Omphalocele is contained within a sac and often associated with chromosomal trisomies (13, 18, 21).

High‑Yield Points - ⚡ Biggest Takeaways

  • First-trimester combined screening integrates PAPP-A, β-hCG, and nuchal translucency ultrasound to assess aneuploidy risk.
  • The second-trimester quad screen measures AFP, estriol, β-hCG, and inhibin A.
  • Cell-free DNA (cfDNA) is a highly sensitive and specific non-invasive screen for common aneuploidies.
  • Elevated maternal serum AFP suggests a neural tube defect; low levels can indicate aneuploidy.
  • Screen for gestational diabetes at 24-28 weeks with a glucose challenge test.
  • Universal GBS screening via rectovaginal culture occurs at 36-38 weeks.

Practice Questions: Prenatal screening

Test your understanding with these related questions

A 32-year-old G1P0 woman undergoes her 2nd-trimester ultrasound in a community hospital. During her prenatal care, she was found to have mild anemia, low levels of folate, and serum alpha-fetoprotein levels greater than 2 multiples of the median (MoM) on 2 separate occasions. Her 1st-trimester ultrasound was significant for the absence of the intracranial lucency, no visualization of the cisterna magna, and posterior shift of the brain stem. These 2nd-trimester ultrasound reports reveal the widening of the lumbosacral spine ossification centers and the presence of a sac in proximity to the lumbosacral defect. Which of the following statements best describes the congenital defect in the fetus?

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Flashcards: Prenatal screening

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Latent tuberculosis infections result in a positive _____ skin test

TAP TO REVEAL ANSWER

Latent tuberculosis infections result in a positive _____ skin test

purified protein derivative (PPD+)

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