Prenatal Testing Methods

Prenatal genetic testing encompasses screening and diagnostic approaches used to assess the risk of or diagnose genetic conditions in a developing fetus. The distinction between screening tests (which estimate probability) and diagnostic tests (which provide definitive answers) is fundamental to genetic counseling and informed consent in prenatal care.

Screening Tests (Non-Invasive)

• Non-invasive prenatal testing (NIPT) / cell-free DNA (cfDNA) screening:

  • Analyzes cell-free fetal DNA (cffDNA) circulating in maternal blood, derived primarily from placental trophoblast cells
  • Available from 10 weeks' gestation onward
  • Fetal fraction: The proportion of total cfDNA that is fetal in origin. Minimum fetal fraction of ~3–4% required for reliable results. Low fetal fraction (associated with early gestational age, high maternal BMI, certain aneuploidies) can cause test failure or inaccurate results.
  • Highest sensitivity/specificity for trisomy 21 (>99% detection rate, ~0.04% false positive rate). Also screens for trisomies 18 and 13, and sex chromosome aneuploidies. Some labs offer expanded panels (microdeletions, rare trisomies), though with lower PPV.
  • It is a screening test, NOT diagnostic: A positive cfDNA result must be confirmed with diagnostic testing (amniocentesis or CVS). The positive predictive value (PPV) depends on the prior probability (maternal age, prevalence of the condition).
  • Confined placental mosaicism (CPM): A major source of false positive and false negative cfDNA results. Since cffDNA originates from the placenta, mosaicism limited to the placenta (not present in the fetus) can produce discordant results.

• Maternal serum screening:

  • First trimester combined screening (11–13+6 weeks): Nuchal translucency (NT) measurement by ultrasound + serum PAPP-A + free beta-hCG. Detection rate ~82–87% for trisomy 21.
  • Quad screen (15–22 weeks): AFP, hCG, unconjugated estriol, inhibin A. Detection rate ~81% for trisomy 21. Also screens for trisomy 18 and open neural tube defects (elevated AFP).
  • Sequential/integrated screening: Combines first and second trimester results for improved detection rates (~90–95%).
  • Key analyte patterns for trisomy 21: Low AFP, high hCG, low uE3, high inhibin A (remember: "21 = low-high-low-high" for the quad screen).

• Ultrasound markers:

  • First trimester: Increased nuchal translucency, absent nasal bone, tricuspid regurgitation, reversed ductus venosus flow
  • Second trimester: Echogenic bowel, short femur/humerus, echogenic intracardiac focus, pyelectasis, ventriculomegaly
  • Structural anomalies (cardiac defects, omphalocele, duodenal atresia) may prompt diagnostic testing

Diagnostic Tests (Invasive)

• Chorionic villus sampling (CVS):

  • Timing: 10–13 weeks' gestation
  • Procedure: Transcervical or transabdominal sampling of chorionic villi (placental tissue)
  • Miscarriage risk: ~0.1–0.2% above baseline (lower than historically quoted)
  • Advantage: Earlier results than amniocentesis, enabling earlier decision-making
  • Limitation: Confined placental mosaicism (~1–2% of CVS samples). Mosaicism found in CVS may not reflect the true fetal karyotype. Amniocentesis may be needed for confirmation.
  • Tests available: Karyotype, CMA, FISH, molecular testing, enzyme assays

• Amniocentesis:

  • Timing: Typically 15–18 weeks' gestation (can be done later)
  • Procedure: Transabdominal needle aspiration of amniotic fluid containing fetal cells (amniocytes)
  • Miscarriage risk: ~0.1–0.3% above baseline
  • Advantage: Directly samples fetal cells (less CPM concern than CVS). Can also assess AFP in amniotic fluid (for neural tube defects) and perform metabolic studies.
  • Limitation: Later timing means results available in mid-second trimester. Culture required for karyotype (10–14 days); FISH provides rapid preliminary results (24–48 hours).
  • Tests available: Karyotype, CMA, FISH, molecular testing, biochemical analysis, AFP level

Comparing CVS and Amniocentesis

• Informed consent is paramount: Genetic counselors must ensure patients understand the difference between screening and diagnostic testing, the limitations of each, and the implications of results before testing.
• Cascade decision-making: A positive screening result (NIPT, serum screen, or ultrasound finding) often leads to offering diagnostic testing. A negative screening result reduces but does not eliminate risk.
• PPV depends on prevalence: A positive cfDNA result for trisomy 21 in a 38-year-old has a much higher PPV than the same result in a 25-year-old, because the baseline prevalence of trisomy 21 increases with maternal age.
• Expanded NIPT panels: Screening for microdeletions (e.g., 22q11.2) and rare aneuploidies has lower PPV than common trisomies. Genetic counselors should discuss the higher false positive rates and limited clinical validation.
• Patient autonomy: Prenatal testing is always optional. Results may be used for pregnancy management, preparation, or termination. The decision belongs to the patient.

• cfDNA is a screening test: Cannot replace diagnostic testing. False positives and false negatives occur, particularly for conditions other than trisomy 21.
• Test failures: Low fetal fraction, maternal obesity, early gestational age, and maternal aneuploidy or malignancy can cause unreliable results or test failure.
• CVS cannot test for neural tube defects: Amniotic fluid AFP is needed; CVS does not provide access to amniotic fluid.
• Neither CVS nor amniocentesis detects all conditions: These procedures provide material for cytogenetic and molecular testing, but the specific tests ordered determine what is detected.
• Maternal cell contamination: Possible in both CVS and amniocentesis samples; can lead to erroneous results, particularly in molecular testing. Laboratories perform maternal cell contamination studies to mitigate this.