P-439 Analysis of Products of Conception (POC) using Whole Genome Next Generation Sequencing (NGS) in pregnancy loss after euploid or mosaic embryo transfer

Abstract Study question What is the outcome of genetic assessment for fetal chromosomal aberrations in miscarriages after euploid/mosaic embryo transfers (ET)? Summary answer POC testing should be recommended in patients with E/M-ET to screen for chromosomal aberrations occurring post-implantation or beyond the detection limit of PGT-A testing What is known already Spontaneous abortion is the most common complication of pregnancy, affecting approximately 15% of clinically recognized pregnancies. Although the etiology is diverse, fetal chromosomal aberrations account for 50-70% of first trimester miscarriages. Assessment of the fetal chromosomal composition is recommended by ACOG guidelines and aids in counselling and management of patients’ reproductive options. The value of this assessment in patients with miscarriage after euploid/mosaic ET has not evaluated. At the same time there is generally a lack of follow-up studies to examine the accuracy of PGT-A testing and investigate the rates of post-implantation errors causing miscarriage. Study design, size, duration This retrospective study had REB approval. This study was performed at the CReATe Fertility Centre and its Reproductive Genetics Laboratory in Toronto, Canada, between January 2019-December 2023.We evaluated the molecular karyotype of POC samples from first trimester <12GW miscarriages after ART: IVF-PGT-group euploid (E) and mosaic (M) embryo transfers (ET) and IVF-group; IUI and natural conception-NC groups; while controlling for maternal cell contamination (MCC) by high-resolution whole genome NGS. Participants/materials, setting, methods 549 POC samples from early miscarriages after fertility treatment were obtained by suction-dilatation-and curettage. Selection of fetal tissue/chorionic villi (4 representative samples for each case) was performed under a dissecting microscope. Maternal and paternal DNA was obtained to test for MCC. MCC was determined by testing maternal/gestational carrier and fetal DNA (fDNA) with highly informative STRs (AmpFLSTR Identifiler kit). Whole genome low-pass NGS (0.1xcoverage) was performed using the Illumina platform and analyzed with NxClinical Software. Main results and the role of chance Fetal tissue was obtained from 90.7% (498/549) of tested samples and the rest had MCC (9.3%, 51/549): IVF-PGT-A(n = 183), IVF(n = 92), IUI(n = 46) and NC(n = 173). The overall mean maternal age was 37±3.2years. The average gestational age at D&C was 8w4d (range 5w-12w6d). Overall, NGS analysis of fDNA showed 44.6% male and 55.3% female fetuses, 57.6% (287/498) were euploid, 39.5% (197/498) were aneuploid and 1.8% (9/498) were mosaic. In IVF-PGT-A group there were 169 POC from euploid ET and 14 from mosaic ET. 98.2% (n = 166/169) of POCs from IVF-PGT-A euploid-ETs were confirmed as euploid, and 3 had confined placental mosaicism (CPM) (trisomy-11, trisomy-4, monosomy-X). POC from mosaic-ET (n = 14), showed CPM in 3/14(21.4%): TE mosaicism was confirmed in 1/14(7.1%), 2/14(14.3%) had CPM differing from the PGT-A result, 11/14(78.6%) were euploid. POC euploidy rate in IVF (no PGT-A) group was 59.8%(55/92), in IUI 34.8%(16/46) and in NC 24.3%(42/173). In NC group CPM was detected in 1.7%(3/173). The frequency of aberrations detected in POC(n = 197) were: T22, 16.5%; T16,12.9%; T15, 11.3%; T20, 5.2%; T8, 3.6%; T7, T9 and T13, 3.1%; T10 and T14, 2.6%; T18 and T21, 2.1%; T12, 1.5%; 1% of each T2, T4 and T11; monosomy-X,6.7% and 11.3% triploid (69,XXY or 69,XXX). Limitations, reasons for caution MCC is relatively high in POC samples from early pregnancies (9.3%) and controlling for it is warranted. Although 4 representative samples of placental tissue were tested, the true extent and existence of CPM may not be captured correctly. Wider implications of the findings NGS analysis of POC from first trimester miscarriages after fertility treatment would improve diagnostic accuracy, and could aid in patient counselling and management. We observed high PGT-A accuracy (100% for gender) and 98.2% for fetal ploidy with CPM rate of 1.8% (which is lower than observed after CVS (4%)). Trial registration number not applicable

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