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"noninvasive prenatal testing" AND aneuploidy

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ACOG current guidance cell-free DNA screening prenatal genetic testing NIPT 2025

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Non-invasive Prenatal Testing (NIPT)

Definition
NIPT, also called cell-free DNA (cfDNA) screening, is a maternal blood test used to screen for fetal chromosomal aneuploidy. It analyzes placental DNA fragments circulating in maternal plasma. It is a screening test, not a diagnostic test.

Principle

  • Placental trophoblast-derived cfDNA is present in maternal blood.
  • A blood sample is taken from the mother, usually from 10 weeks of gestation onward.
  • Massive parallel sequencing or SNP-based analysis measures relative amounts of selected chromosomes.
  • Over-representation or under-representation of chromosomal material indicates a high risk of aneuploidy.
Tietz Textbook of Laboratory Medicine, 7th ed., p. 2757.

Indications

NIPT may be offered to all pregnant women after appropriate pre-test counselling, particularly in:
  • Advanced maternal age
  • Previous fetus or child with aneuploidy
  • Positive first- or second-trimester biochemical screening
  • Ultrasound marker suggestive of aneuploidy
  • Parental balanced chromosomal rearrangement
  • Maternal anxiety or preference for a highly accurate screening test.

Conditions screened

Most validated for:
  • Trisomy 21 - Down syndrome
  • Trisomy 18 - Edwards syndrome
  • Trisomy 13 - Patau syndrome
  • Sex chromosome aneuploidies: Turner syndrome (45,X), Klinefelter syndrome (47,XXY), etc.
  • Fetal sex determination
Some platforms additionally offer screening for selected microdeletions and rare autosomal trisomies, but these have lower predictive value and should be interpreted cautiously.

Methods

  1. Massively parallel shotgun sequencing
    • Counts DNA fragments mapping to chromosomes 13, 18, and 21.
    • An excess of fragments from a chromosome suggests trisomy.
  2. SNP-based method
    • Uses polymorphic SNPs to distinguish maternal and fetal DNA.
    • Can help identify triploidy and determine parental origin in selected situations.
Tietz Textbook of Laboratory Medicine, 7th ed., p. 2757.

Advantages

  • Non-invasive: only maternal venous blood is needed.
  • No procedure-related risk of miscarriage.
  • Can be performed early, from about 10 weeks.
  • Highest sensitivity and specificity among screening tests for common trisomies, especially trisomy 21.
  • Reduces need for invasive procedures such as chorionic villus sampling (CVS) and amniocentesis.
  • Faster reporting than conventional karyotyping.

Limitations

  • It is not diagnostic. A high-risk result must be confirmed by CVS or amniocentesis with karyotyping/microarray.
  • Does not exclude all genetic, structural, or congenital abnormalities.
  • “No-call” or inconclusive results can occur, commonly due to low fetal fraction.
  • Accuracy is reduced in obesity, early gestation, multiple pregnancy, vanishing twin, maternal chromosomal abnormality, malignancy, or placental mosaicism.
  • False-positive results may occur due to confined placental mosaicism, vanishing twin, maternal copy-number variants, or maternal malignancy.
  • A negative result does not replace routine anomaly scanning.

Interpretation and counselling

  • Explain that NIPT provides a risk estimate, not confirmation of disease.
  • A screen-positive result requires genetic counselling and confirmatory invasive prenatal diagnosis.
  • If fetal structural anomaly or increased nuchal translucency is detected on ultrasound, diagnostic testing is preferred even if NIPT is negative.
  • Routine first-trimester ultrasound and detailed anomaly scan remain necessary.

Conclusion

NIPT is a safe, early, and highly accurate screening method for common fetal aneuploidies, especially trisomy 21. Its major limitation is that it cannot establish a diagnosis; abnormal results must always be confirmed by invasive diagnostic testing.
Current ACOG patient guidance also states that cfDNA comes largely from the placenta, has no direct fetal procedural risk, and an increased-risk screen should be followed by diagnostic testing, as summarized in ACOG genetic-testing guidance.
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