OC11.02: A late first trimester scan can alter management after a high‐risk NIPT result

To evaluate the impact of a detailed late first trimester ultrasound (LTFU) on the positive predictive value (PPV) of a high-risk non-invasive test (NIPT) result for various aneuploidies. A retrospective study of all cases undergoing invasive prenatal testing from 3 tertiary providers of obstetric ultrasound over 4 years, each using genome wide NIPT as a first line screening test. An amniocentesis or chorionic villus sampling (CVS) was performed on 2657 patients, 51% having had a NIPT, 612 (45%) with a high-risk result. The LTFU significantly altered the PPV of the NIPT for Trisomies 13,18, 21, as well as for 45X and rare autosomal trisomies, but not for other sex chromosomal abnormalities or segmental imbalances (> 7 Mb). The more severe chromosomal abnormalities had the largest alteration to the PPV. After a normal LTFU, the incidence of a mosaic CVS result was higher for Trisomy 13 than for Trisomy 21. A LTFU after a high-risk NIPT result can alter the PPV of many chromosomal abnormalities, assisting counselling regarding invasive prenatal testing and can be reassuring while awaiting an amniocentesis to avoid potential mosaic CVS result.