Everything You Need to Know About Edwards syndrome (Trisomy 18) for Step 1

Edwards syndrome (trisomy 18) is one of those Step 1 “you either recognize it instantly or you don’t” diagnoses—because the clues are so classic: severe growth restriction, clenched hands with overlapping fingers, rocker-bottom feet, and congenital heart disease. The trick is going beyond the buzzwords: know why it happens, how it presents, how it’s diagnosed, and what questions the NBME loves to ask around maternal age, prenatal screening, and prognosis.

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Big Picture: What Is Edwards Syndrome?

Edwards syndrome = Trisomy 18, a chromosomal aneuploidy with an extra copy of chromosome 18.

• Karyotype: $47,XX,+18$ or $47,XY,+18$
• Epidemiology:
  • Risk increases with advanced maternal age
  • Much less common than trisomy 21, but high mortality
• Prognosis (very high-yield):
  • Many pregnancies end in fetal loss
  • Of liveborn infants, majority die within the first year (often within weeks to months) due to cardiopulmonary complications

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First Aid cross-reference: Genetics → Chromosomal disorders (Edwards syndrome / Trisomy 18)

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Pathophysiology: How Trisomy 18 Happens

1) Meiotic nondisjunction (most common)

• Usually maternal meiosis I nondisjunction
• Leads to a full extra chromosome 18 in most cells

2) Mosaic trisomy 18

• Post-zygotic mitotic nondisjunction
• Variable severity; generally milder phenotype depending on fraction of affected cells

3) Translocation-related trisomy 18 (rare)

• Less tested than translocation Down syndrome, but conceptually possible

Core mechanism to remember: gene dosage imbalance → widespread developmental disruption, especially affecting cardiac, craniofacial, and limb development.

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Clinical Presentation: The Classic Edwards Syndrome Picture

Think: Growth restriction + craniofacial anomalies + limb positioning abnormalities + congenital heart disease.

High-yield findings (Step 1 must-know)

• Severe intellectual disability
• Growth restriction (prenatal and postnatal)
• Prominent occiput
• Micrognathia (small jaw)
• Low-set ears
• Clenched hands with overlapping fingers (very classic)
• Rocker-bottom feet
• Congenital heart defects
  • Common: VSD, PDA, other complex congenital heart disease
• Renal abnormalities may occur (e.g., horseshoe kidney can show up across aneuploidies; renal malformations are common in trisomy 18 broadly)

Quick “spot diagnosis” comparison table

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Diagnosis: Prenatal Screening and Confirmatory Testing

Step questions often test the difference between screening vs diagnostic tests and how trisomy 18 affects serum markers.

Prenatal screening (suggests risk, not definitive)

First trimester

• Increased nuchal translucency (nonspecific—also seen in Down, Turner, cardiac defects)

Second trimester “quad screen” pattern (high-yield)

Trisomy 18 classically shows everything low:

• ↓ AFP
• ↓ β-hCG
• ↓ Estriol
• ↓ Inhibin A (often decreased as well)

Memory hook: Edwards = “E for Everything low.”

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First Aid cross-reference: Reproductive → Prenatal testing; Genetics → Aneuploidy screening patterns

Ultrasound clues (commonly tested)

• IUGR
• Congenital heart defects
• Clenched hands / overlapping fingers
• Rocker-bottom feet
• Craniofacial abnormalities (micrognathia, prominent occiput)

Confirmatory (diagnostic) testing

You confirm trisomy 18 with fetal genetic testing:

• Chorionic villus sampling (CVS) (earlier)
• Amniocentesis (later)
• Karyotype (classic), FISH, or chromosomal microarray (more resolution)

Cell-free DNA (cfDNA / NIPT)

• Very good screening test for common aneuploidies (including trisomy 18)
• Still requires confirmatory invasive testing if positive (test nuance NBME likes)

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Management: What Do You Do After Diagnosis?

Prenatal counseling (high yield concept)

• Discuss prognosis and options with family
• Coordinate with maternal-fetal medicine and neonatology/palliative care as appropriate

Postnatal care (supportive, individualized)

There is no curative treatment for the genetic cause. Management is largely:

• Supportive care (feeding support, respiratory support)
• Treatment of complications:
  • Management of congenital heart disease (often not fully correctable depending on severity)
  • Seizure management if present
  • Addressing apnea, aspiration risk, infections

Prognosis-focused care

Because mortality is high, many scenarios emphasize:

• Goals-of-care conversations
• Palliative approach depending on severity and family preferences

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High-Yield USMLE Associations & Testable Patterns

1) Advanced maternal age → increased nondisjunction risk

• Trisomy 18, 21, 13 all increase with maternal age
• On exams: older pregnant patient + abnormal screen + classic ultrasound = pick the aneuploidy

2) “Everything low” quad screen

If a question gives:

• Low AFP + low β-hCG + low estriol → think trisomy 18

(Contrast: Down syndrome typically has ↓ AFP, ↑ β-hCG, ↓ estriol, ↑ inhibin A.)

3) Buzzword clustering

• Overlapping fingers + rocker-bottom feet is a near-direct arrow to trisomy 18.
• Prominent occiput is another strong discriminator.

4) Survival/prognosis is part of diagnosis

NBME-style vignettes may ask what you tell the parents:

• Expect severe developmental impairment
• High infant mortality, often from cardiopulmonary complications

5) Know the testing ladder

• Screening (quad, cfDNA, ultrasound) → confirm with CVS/amnio and karyotype/microarray

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Rapid-Fire Review (One-Minute Drill)

Edwards syndrome (Trisomy 18):

• Cause: meiotic nondisjunction (usually maternal)
• Screen: quad screen all low (AFP, β-hCG, estriol ± inhibin A)
• Findings: IUGR, prominent occiput, micrognathia, low-set ears, clenched hands with overlapping fingers, rocker-bottom feet, congenital heart disease (VSD/PDA)
• Diagnosis: CVS/amnio + karyotype/FISH/microarray
• Management: supportive + complication-based; often palliative focus
• Prognosis: high mortality within first year

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Common Exam Traps (Don’t Miss These)

• Positive NIPT is not definitive → still confirm with CVS/amnio.
• Rocker-bottom feet can appear in other conditions, but rocker-bottom + overlapping fingers + prominent occiput strongly favors trisomy 18.
• If the stem emphasizes cleft lip/palate + polydactyly, that’s more consistent with Patau (trisomy 13).