EP18.07: Repeat cell‐free DNA testing in the third trimester after discordant high‐risk results

A substantial proportion of cell-free DNA (cfDNA) screening results that are discordant to fetal diagnostic investigations might be attributable to confined placental mosaicism (CPM). In this ongoing study, we aim to investigate the role of repeated cfDNA screening in the third trimester of pregnancy in predicting the likelihood of CPM after a ‘false-positive’ screening result. Pregnant women at participating practices in Melbourne, Australia, who were identified as having had high-risk cfDNA screening which was discordant with euploid fetal diagnostic investigations were offered a repeat cfDNA test at 31-33 weeks' gestation. Maternal plasma samples were collected and analysed by whole-genome massively parallel sequencing (MPS). After delivery, biopsies were taken from each quadrant of the placenta, combined and analysed by microarray. We present our preliminary findings in table 1. Case N6 who's placental microarray results were euploid despite repeated high-risk cfDNA results was offered an additional cfDNA screen postnatally to investigate for a maternal origin, the results of which are pending at the time of this abstract submission. These preliminary findings support the hypothesis that CPM is a substantial contributor to discordant cfDNA results. cfDNA results are not necessarily concordant with placental genotypes, even when repeated at later gestations in pregnancy.