numbersaneuploidiescan

Numbers Aneuploidy Scan (NAS) is a non-invasive prenatal screening method designed to detect fetal aneuploidies by counting chromosomal DNA fragments circulating in maternal blood. The technique evaluates the relative representation of chromosome sequences, typically focusing on chromosomes commonly involved in aneuploidies such as 21, 18, and 13. Data are collected through sequencing or targeted amplification and then analyzed with statistical or machine learning models to compare observed counts against expected norms for gestational age and fetal fraction.

NAS is intended to indicate the likelihood of a fetal aneuploidy rather than provide a definitive diagnosis. A positive or high-risk result is usually followed by diagnostic testing, such as amniocentesis or chorionic villus sampling, to confirm fetal karyotype. A negative result reduces risk but does not guarantee absence of aneuploidy, particularly for less common chromosomal abnormalities or confined placental mosaicism.

Benefits of NAS include its non-invasiveness, relatively high sensitivity for the well-characterized trisomies, and rapid reporting. Limitations include false positives from maternal mosaicism or placental discrepancies, the requirement for a sufficient fetal fraction, and the possibility of inconclusive results. Ethical considerations involve incidental findings and informed consent.

NAS is part of the broader family of non-invasive prenatal tests and complements ultrasound screening and invasive diagnostic options. The term NAS may be used variably in literature, and implementation depends on clinical guidelines and regulatory approval.