Management of Atypical NIPT Result for Trisomy 21 with Reassuring Ultrasound Findings

Understanding “Atypical” NIPT Results

An atypical (indeterminate) cell‑free DNA screening result reflects technical limitations such as low fetal fraction or ambiguous signal rather than an elevated risk for trisomy 21, and therefore has a markedly lower positive predictive value than a clearly high‑risk screen. 1

A nuchal translucency (NT) measurement < 3 mm in the first trimester is highly reassuring; approximately one‑third of fetuses with NT ≥ 3 mm have chromosomal abnormalities, whereas a NT of 2 mm falls well below this risk threshold. 2
The presence of a well‑formed nasal bone in the first trimester is a strong negative predictor for Down syndrome; absent or hypoplastic nasal bone occurs in 58–64 % of trisomy 21 fetuses but in < 1 % of chromosomally normal fetuses, thus its presence substantially lowers risk. 1
A normal detailed anatomy scan at 16 weeks (absence of soft markers such as echogenic intracardiac focus, shortened long bones, echogenic bowel, or thickened nuchal fold) reduces the probability of trisomy 21 because 50–75 % of affected fetuses exhibit detectable structural anomalies by this gestational age. 2
Combining a normal NT, a present nasal bone, and a flawless mid‑trimester anatomy scan reduces the baseline age‑related risk for trisomy 21 (≈ 1 in 1,000 at 28 years) by roughly 90–95 %, yielding a residual risk estimated to be < 1 in 10,000. 8

The Society for Maternal‑Fetal Medicine (SMFM) advises that diagnostic testing is not required for isolated soft‑marker findings when a negative cfDNA screen is obtained; however, an atypical NIPT result does not constitute a negative screen and therefore warrants a different discussion. [4][5]
Counseling should emphasize that, despite reassuring ultrasound markers, the atypical NIPT leaves a residual uncertainty that can only be resolved definitively by invasive diagnostic testing. [4][1]

Amniocentesis remains the gold‑standard invasive test for definitive chromosomal diagnosis and should be offered when an atypical NIPT result persists. 1
Chromosomal microarray performed on amniotic fluid can identify trisomy 21 as well as other aneuploidies and clinically significant copy‑number variants that standard NIPT may miss. 2
The procedure‑related risk of pregnancy loss from amniocentesis is estimated at 1 in 300–500, a factor that must be balanced against the remaining uncertainty about fetal karyotype. 1

If the patient declines invasive testing, a repeat cell‑free DNA screen or a second‑trimester quad screen can be considered for additional risk stratification. [4][5]1
A quad screen detects approximately 80–85 % of trisomy 21 cases, which is lower than the detection rate of first‑trimester combined screening, and therefore provides less reassurance than a repeat cfDNA test. 8

Standard third‑trimester growth monitoring and routine prenatal care are sufficient; no extra specialized surveillance is indicated based solely on an atypical NIPT result coupled with reassuring ultrasound findings. [4][6]

Do not dismiss an atypical NIPT result solely because ultrasound markers are normal; definitive diagnosis still requires karyotyping when complete reassurance is desired. 1
Do not assume that normal ultrasound eliminates all genetic risk, as both NIPT and imaging can miss certain chromosomal abnormalities, microdeletions, and structural variants. 2
Do not equate an “atypical” result with a “positive” result; the former usually reflects technical issues rather than fetal pathology and should be communicated accordingly to reduce patient anxiety. 1
Do not withhold the option of diagnostic testing; offering amniocentesis respects patient autonomy and provides the only path to absolute certainty. 1