NIPT: what it covers, what it doesn't, and how to counsel patients

Non-invasive prenatal testing (NIPT) analyses fragments of cell-free fetal DNA (cffDNA) circulating in maternal blood to assess the risk of chromosomal conditions in the pregnancy. NIPT is a screening test, not a diagnostic test. Understanding this distinction — and communicating it clearly to patients before and after testing — is the most important aspect of NIPT counselling.

Standard NIPT panels screen for the three most clinically significant autosomal trisomies: trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). Most commercially available panels also include sex chromosome aneuploidies (e.g., Turner syndrome, Klinefelter syndrome, triple X, and 47,XYY). Extended or premium panels add selected microdeletions and microduplications, additional autosomal trisomies, and in some cases a focused maternal pathogen screen. Clinicians should be explicit about which conditions are and are not covered by the specific panel ordered.

NIPT does not screen for all chromosomal conditions. It does not reliably detect structural chromosomal rearrangements (balanced translocations, inversions), does not screen for single-gene (Mendelian) disorders unless the panel is specifically designed to do so, and does not assess fetal anatomy or neural tube defects. Patients who associate 'genetic testing in pregnancy' with a comprehensive assessment of fetal health need clear pre-test counselling that NIPT has a defined and limited scope.

The positive predictive value (PPV) of NIPT — the probability that a high-risk result reflects a truly affected pregnancy — depends heavily on the prevalence of the condition in the tested population. For trisomy 21 in a 35-year-old, PPV may be high; for microdeletion syndromes in a low-risk population, PPV can be considerably lower. Every positive or high-risk NIPT result must be confirmed by diagnostic testing (chorionic villus sampling or amniocentesis) before any irreversible clinical decision is made. No management change should follow a positive NIPT result alone.

Fetal fraction — the proportion of cell-free DNA in the maternal sample that is of fetal origin — is a key quality metric. Low fetal fraction, which is more common at earlier gestational ages and in patients with higher BMI, can result in a failed or inconclusive result. When this occurs, options include a redraw at a later gestational age or proceeding directly to diagnostic testing if timing is critical.

Before NIPT, patients benefit from a brief counselling conversation covering what the test screens for, the meaning of a high-risk versus low-risk result, what a failed or inconclusive result means, and the difference between NIPT and diagnostic testing. After a high-risk result, prompt referral to a clinical genetics service or maternal-fetal medicine specialist is recommended to review the result in full clinical context, discuss confirmatory testing options, and provide support before any decisions are made.