O-076 Update on the International Registry of Mosaic Embryo Transfers

Abstract Study question Which are the results obtained by the International Registry of Mosaic Embryo Transfers? Summary answer An update of clinical outcomes and post-natal results of mosaic embryos with low and high-level of mosaicism is provided. What is known already Chromosomal mosaic embryos are characterized by the presence of chromosomally different cell lines within the same embryo. While the transfer of these embryos is now offered as an option for women who undergo in vitro fertilization (IVF), several concerns remain. For instance, the limited data on pregnancy outcome and the possibility that intra-biopsy mosaicism in the TE is a poor predictor of the ploidy status of the ICM. Therefore, some argue that mosaicism should be not reported until a clear classification of such embryos in relation to their reproductive potential has been defined. Study design, size, duration We collected the clinical outcomes of 2045 mosaic embryos transferred in women who undergoing IVF between May 2019-May 2022. All embryos were cultured to blastocyst stage; trophectoderm (TE) biopsy was performed on Day-5 of development or Day6/7 for slow-growing embryos. The clinical outcomes obtained after the transfer of mosaic embryos with the different chromosomal constitutions were compared. Prenatal and post-natal outcome was collected for available cases. Participants/materials, setting, methods Preimplantation genetic testing for aneuploidies (PGT-A) was performed using high-resolution next-generation sequencing (NGS) methodology. TE biopsies were classified as mosaic if they had 20%-80% abnormal cells. For statistical analysis, mosaic embryos were divided into groups based on mosaic levels and chromosomal constitution detected in TE: single mosaic aneuploidy (monosomy/trisomy; SM), double mosaic chromosomes (monosomy/trisomy or combination, DM), complex mosaic aneuploidy (>2 different aneuploidies; CM) and mosaic segmental aneuploidy (single and double deletion/insertion >5Mb, MS). Main results and the role of chance Embryos classified as ‘low-mosaic’ by NGS-based PGT-A have a higher likelihood of achieving implantation compared to ‘high-mosaic’ embryos (48% vs. 39.%; p < 0.05 ), as well as ongoing pregnancy/live birth (40% vs. 29%; p < 0.05). Chromosomal composition of mosaicism abnormalities dictates the success rate of mosaic embryo transfers, with low and high segmental mosaics being preferable over low- and high-mosaics involving whole chromosomes. For 550/670 pregnancies, parental tests and post-natal data were available. The majority (99.75%) of the babies were largely healthy by routine physical inspection by neonatologists (no gross abnormalities in babies from mosaic embryos, n = 495). Prenatal testing performed on pregnancies from mosaic embryo transfers were generally normal. The mosaicism detected at the embryonic stage by PGT-A was reflected in prenatal testing in only 5 out of 550 pregnancies (0.9%) in which the mosaicism identified with PGT-A at the blastocyst stage was reflected in gestation by prenatal chromosomal testing as true fetal mosaicism. Limitations, reasons for caution Additional clinical data must be obtained to evaluate the contribution of each different chromosome before this approach can be evaluated as an additional tool to choose mosaic embryos for transfer. Wider implications of the findings The International Registry of Mosaic Embryo transfers continues to grow in sample size, in turn increasing the power of analysis. The findings of the mosaic embryo transfer registry can help educate the management and selection of embryos in the clinic. Trial registration number Not applicable