What are the recommended prenatal screening and diagnostic tests for chromosomal abnormalities and neural‑tube defects, including first‑trimester combined testing, cell‑free DNA (non‑invasive prenatal testing), quad screen, chorionic villus sampling, and amniocentesis, and what are their indications and risks?

Prenatal Testing for Step 2 CK Exam

All pregnant women should be offered both screening and diagnostic testing options for fetal aneuploidy and neural tube defects, with first-trimester combined screening (NT, PAPP-A, hCG at 11-14 weeks) being the preferred initial approach for those presenting early, followed by MSAFP screening at 16-18 weeks for neural tube defects.

Screening Tests Overview

First-Trimester Combined Screening (11-14 weeks)

This is the optimal initial screening approach for women presenting before 14 weeks' gestation 1. The test combines:

• Nuchal translucency (NT) measurement via ultrasound (fluid-filled space behind fetal neck)
• PAPP-A (pregnancy-associated plasma protein A)
• Free beta-hCG or intact hCG

This combination achieves approximately 70% detection rate for Down syndrome with a 5% false-positive rate 1. The NT measurement alone has similar performance but combining with biochemical markers improves accuracy.

Critical technical requirement: NT measurement requires strict quality control, proper sonographer training, and certification through available organizations 1. Without these standards, the test should not be offered.

Key clinical pearl: An NT measurement ≥3.5 mm warrants detailed anatomic ultrasound and echocardiogram even if aneuploidy screening is low-risk or karyotype is normal, as this finding associates with congenital heart defects, diaphragmatic hernias, skeletal dysplasias, and genetic syndromes 1.

Second-Trimester Screening (15-20 weeks)

Quad Screen includes:

• AFP (alpha-fetoprotein)
• hCG
• uE3 (unconjugated estriol)
• Inhibin-A

Triple screen omits inhibin-A but is less sensitive. The quad screen should be offered to women presenting in the second trimester or those who declined first-trimester screening 2.

Optimal timing: 16-18 weeks' gestation 2.

MSAFP for Neural Tube Defects

All women should be offered MSAFP screening at 16-18 weeks, even if they had first-trimester screening or CVS 2, 1. This is the only screening test that detects open neural tube defects and anencephaly. Cell-free DNA does not screen for neural tube defects 3.

Cell-Free DNA (cfDNA/NIPT)

Cell-free DNA has high sensitivity and specificity for trisomies 21,18, and 13 3. However, important limitations exist:

• "No call" results (indeterminate/uninterpretable) indicate increased aneuploidy risk and require genetic counseling, comprehensive ultrasound, and diagnostic testing 3
• Does not replace diagnostic testing - positive results require confirmation with CVS or amniocentesis before any pregnancy management decisions 3
• Does not screen for neural tube defects - MSAFP or ultrasound still needed 3
• Not a diagnostic test - cannot be used alone for clinical decisions including termination 3

Diagnostic Tests

Chorionic Villus Sampling (CVS)

Timing: 11+0 to 13+6 weeks (can be done 14+0 to 14+6 weeks with individualized counseling) 4

• Do not perform before 10+0 weeks 4
• Preferably perform from 11+0 weeks onward to reduce technical challenges 4

Miscarriage risk: <0.5% when performed by appropriately trained operator 4

Advantages: Earlier diagnosis, allows earlier decision-making

Special consideration for cfDNA results: For chromosomes with high confined placental mosaicism rates (trisomy 13, monosomy X, rare autosomal trisomies), consider amniocentesis instead if first-trimester ultrasound is normal 5. For chromosomes with imprinted genes (6,7,11,14,15,20), CVS should be performed first, followed by amniocentesis if abnormal 5.

Amniocentesis

Timing: From 15+0 weeks onward 4

• Do not perform before 15+0 weeks 4

Miscarriage risk: <0.5% when performed by appropriately trained operator 4

Advantages: Lower confined placental mosaicism risk, can be combined with AFP measurement for neural tube defect assessment

Clinical Algorithm for Step 2 CK

For Women Presenting <14 Weeks:

1. Offer first-trimester combined screening (NT + PAPP-A + hCG) at 11-14 weeks 1
2. Follow with MSAFP screening at 16-18 weeks for neural tube defects 1
3. If screen positive → offer genetic counseling and amniocentesis 2

For Women Presenting ≥14 Weeks:

1. Offer quad screen at 15-20 weeks (optimal 16-18 weeks) 2
2. If screen positive → offer genetic counseling and amniocentesis 2

For Women ≥35 Years Old:

CVS and amniocentesis should be offered directly for diagnosis of aneuploidy 2. However, age is no longer used as an absolute cutoff - women should be informed of their adjusted risk and allowed to make decisions based on individual risk/benefit assessment 1.

For Twin Pregnancies:

• First-trimester screening can be used but has limitations 1
• Miscarriage risk for invasive testing is approximately 1% (higher than singletons) 4

Critical Pitfalls to Avoid

1. Gestational age accuracy is paramount: If gestational age changes by ≥2 weeks after ultrasound, reinterpret all screening results 2. If sample drawn <15 weeks, send new sample with corrected age 2.

2. Never make management decisions on screening alone: All positive screening tests require diagnostic confirmation before pregnancy termination or other major decisions 3.

3. Don't forget neural tube defect screening: Women who have first-trimester screening or CVS still need MSAFP or anatomic ultrasound at 16-20 weeks 1.

4. Integrated screening requires non-disclosure: If offering integrated screening (first + second trimester combined), ensure adequate follow-up and confirm patient accepts delayed results 1.

5. Rhesus status: Non-sensitized Rh-negative women need anti-D prophylaxis after invasive procedures 4.

6. Viral screening: Review blood-borne virus screening, viral load, and antigen results before invasive testing; discuss individualized transmission risk 4.