  Non-invasive prenatal testing or NIPT is currently among the top researched
topic in obstetric care. While the performance of the current state-of-the-art
NIPT solutions achieve high sensitivity and specificity, they still struggle
with a considerable number of samples that cannot be concluded with certainty.
Such uninformative results are often subject to repeated blood sampling and
re-analysis, usually after two weeks, and this period may cause a stress to the
future mothers as well as increase the overall cost of the test. We propose a
supplementary method to traditional z-scores to reduce the number of such
uninformative calls. The method is based on a novel analysis of the length
profile of circulating cell free DNA which compares the change in such profiles
when random-based and length-based elimination of some fragments is performed.
The proposed method is not as accurate as the standard z-score; however, our
results suggest that combination of these two independent methods correctly
resolves a substantial portion of healthy samples with an uninformative result.
Additionally, we discuss how the proposed method can be used to identify
maternal aberrations, thus reducing the risk of false positive and false
negative calls.
  Keywords: Next-generation sequencing, Cell-free DNA, Uninformative result,
Method, Trisomy, Prenatal testing
