What causes false positive NIPT results?

A false positive means the test reported high risk but the baby does not have the condition. It is more common than most people expect, particularly for less common conditions. The main causes, per ACMG: Confined placental mosaicism. NIPT analyses DNA from the placenta. Occasionally the placenta carries a chromosome abnormality that the baby does not. This is the commonest explanation. Vanishing twin. Where a twin stopped developing early in pregnancy, its DNA can persist in your blood and produce a positive result. Maternal factors. Because your own DNA is also in the sample, an unsuspected chromosome difference in you can produce a positive result about you rather than your baby. Maternal malignancy. Rarely, an undiagnosed cancer can produce abnormal patterns. ACMG notes this is most likely when NIPT shows multiple aneuploidies or an autosomal monosomy not confirmed in the fetus. Around 25 to 27 in every 100,000 pregnant women have a malignancy diagnosed during pregnancy. There is also the statistical reason, which is not an error at all: for uncommon conditions, even a very accurate test produces a substantial proportion of positive results that are false, simply because the condition is rare. This is why ACMG recommends confirmatory testing by CVS or amniocentesis after a positive result, rather than acting on the screening result. Ask for the positive predictive value for your specific result and age.