Prenatal diagnostic testing and atypical chromosome abnormalities following combined first‐trimester screening: implications for contingent models of non‐invasive prenatal testing

Objectives To investigate by means of a population‐based analysis of a cohort of women who underwent combined first‐trimester screening (CFTS), changes in uptake of invasive prenatal diagnosis according to risk of trisomy 21 (T21) on CFTS, and prevalence and methods for ascertainment of atypical chromosome abnormalities. Methods This was a retrospective cohort study using state‐wide prenatal datasets from Victoria, Australia. A three‐step approach was taken to analyze the data: (1) linkage of records between serum screening and diagnostic results; (2) comparison of rates of diagnostic testing according to CFTS T21 risk result category in a 2014–2015 cohort with those of a historical 2002–2004 cohort; (3) detailed analysis of atypical abnormalities in the 2014–2015 group according to CFTS T21 risk result, individual serum analyte level and other indications for invasive diagnostic testing. Results In 2014–2015, there were 100 418 CFTS results issued for 146 776 births (68.4%). The overall prevalence of atypical chromosome abnormalities in the entire CFTS cohort was 0.10% and was highest in those with CFTS T21 risk > 1 in 10 (4.6%), or serum analyte levels