NIPT Accuracy — Detection Rates and Limitations

Detection rates by condition

For trisomy 21 (Down syndrome), NIPT detects over 99% of affected pregnancies. Trisomy 18 detection is above 97%, and trisomy 13 detection is approximately 90–97%. Sex chromosome abnormality detection is somewhat lower, and microdeletion panels vary more.

These rates far exceed traditional screening: the double marker detects roughly 85–90% of Down syndrome, and the second-trimester quad screen about 81%.

False positives and false negatives

False positives are rare — under 0.1% for trisomy 21 — but they happen, mainly because the DNA tested comes from the placenta rather than the fetus itself (confined placental mosaicism), a vanishing twin, or rarely a maternal chromosomal variation.

False negatives are rarer still but possible — typically from low fetal fraction or unusual mosaicism. This is why positive results are confirmed diagnostically and ultrasound surveillance continues regardless.

The "no-call" result

In a few percent of cases, NIPT returns no result — usually because fetal fraction was too low (early gestation, higher maternal weight). A redraw 1–2 weeks later usually succeeds.

Screening vs diagnosis — the key distinction

NIPT is the best screening test, but it remains a screen: it reports probability, not certainty. Only diagnostic tests — CVS or amniocentesis with karyotype or microarray — examine fetal chromosomes directly and give a definitive answer.