Can NIPT be wrong?

Yes, in both directions, and understanding why helps you interpret your own result. The root cause is that NIPT analyses DNA from the placenta, not from the baby. Usually they match. Sometimes they do not. False positives — the test says high risk, the baby is unaffected. Causes include confined placental mosaicism, where the abnormal chromosomes are present in the placenta but not in the baby, and in theory a vanishing twin, where DNA from a twin that stopped developing early is still present. False negatives — the test says low risk, the baby is affected. Less common, but possible. There is one further possibility worth knowing. Because NIPT analyses all the DNA in your blood, including your own, it can occasionally pick up something about you rather than your baby — an unsuspected chromosome difference in you, or very rarely a sign of a maternal cancer. ACMG notes that the chance of an underlying maternal malignancy is highest when NIPT shows multiple aneuploidies or an autosomal monosomy that is not confirmed in the baby. Around 25 to 27 in every 100,000 pregnant women have a cancer diagnosed during pregnancy. This is uncommon, but it is a real reason results are sometimes followed up in an unexpected direction. This is why a high-risk result is followed by a diagnostic test rather than acted upon directly, and why ACMG recommends that if you decline invasive testing, cord blood is obtained after birth to confirm.