What causes false negative NIPT results?

A false negative means the test reported low risk but the baby does have the condition. It is less common than a false positive, but it happens. The underlying reason is the same: NIPT analyses DNA from the placenta rather than from the baby. Where the placenta and baby differ, the result can be misleading in either direction. Other contributors: a low proportion of placental DNA in the sample, and technical limitations of the particular platform used. More important than the false negative rate, though, is what NIPT does not test for at all. A low-risk result is often understood as "everything is fine", and it is not that. A low-risk NIPT does not exclude: Around half of the chromosome abnormalities amniocentesis would find — around 75% in women under 35 and 43% in women over 35. Unbalanced translocations, deletions and duplications. Single gene conditions such as thalassaemia, cystic fibrosis or sickle cell disease. Open neural tube defects — a separate blood test at 15 to 20 weeks screens for these. Structural abnormalities, which your anomaly scan covers. ACMG specifies that counselling after a negative result should emphasise the reduction, but not elimination, of risk — and that it does not reduce the risk of conditions other than those screened. So continue with your routine antenatal care. The anomaly scan around 18 to 20 weeks remains important regardless of your NIPT result.