One hundred mosaic embryos transferred prospectively in a single clinic: exploring when and why they result in healthy pregnancies.

OBJECTIVE: To investigate the parameters of mosaicism and the biological mechanisms leading to healthy pregnancies from mosaic embryo transfers. DESIGN: Prospective study. SETTING: IVF center and associated research laboratory. PATIENT(S): Fifty-nine patients. INTERVENTION(S): Embryos underwent blastocyst-stage preimplantation genetic testing for aneuploidy by next-generation sequencing. Trophectoderm biopsies containing 20%-80% abnormal cells were deemed mosaic, and corresponding blastocysts were transferred. Mosaic embryos donated to research were examined for karyotype concordance in multiple biopsies and assessed for cell proliferation and death by immunofluorescence and computational quantitation. MAIN OUTCOME MEASURE(S): Chemical start of pregnancy, implantation, fetal heartbeat, and birth. RESULT(S): Globally, mosaic embryos showed inferior clinical outcomes compared with euploid embryos. Aneuploid cell percentage in trophectoderm biopsies did not correlate with outcomes, but type of mosaicism did, as embryos with single mosaic segmental aneuploidies fared better than all other types. Mosaic blastocysts generated from oocytes retrieved at young maternal ages (≤34 years) showed better outcomes than those retrieved at older maternal ages. Mosaic embryos displayed low rates of karyotype concordance between multiple biopsies and showed significant elevation of cell proliferation and death compared with euploid embryos. CONCLUSION(S): After euploid embryos, mosaic embryos can be considered for transfer, prioritizing those of the single segmental mosaic type. If a patient has mosaic embryos available that were generated at different ages, preference should be given to those made at younger ages. Intrablastocyst karyotype discordance and differential cell proliferation and death might be reasons that embryos classified as mosaic can result in healthy pregnancies and babies.

Assessment of aneuploidy concordance between clinical trophectoderm biopsy and blastocyst.

STUDY QUESTION: Is a clinical trophectoderm (TE) biopsy a suitable predictor of chromosomal aneuploidy in blastocysts? SUMMARY ANSWER: In the analyzed group of blastocysts, a clinical TE biopsy was an excellent representative of blastocyst karyotype in cases of whole chromosome aneuploidy, but in cases of only segmental (sub-chromosomal) aneuploidy, a TE biopsy was a poor representative of blastocyst karyotype. WHAT IS KNOWN ALREADY: Due to the phenomenon of chromosomal mosaicism, concern has been expressed about the possibility of discarding blastocysts classified as aneuploid by preimplantation genetic testing for aneuploidy (PGT-A) that in fact contain a euploid inner cell mass (ICM). Previously published studies investigating karyotype concordance between TE and ICM have examined small sample sizes and/or have utilized chromosomal analysis technologies superseded by Next Generation Sequencing (NGS). It is also known that blastocysts classified as mosaic by PGT-A can result in healthy births. TE re-biopsy of embryos classified as aneuploid can potentially uncover new instances of mosaicism, but the frequency of such blastocysts is currently unknown. STUDY DESIGN, SIZE, DURATION: For this study, 45 patients donated 100 blastocysts classified as uniform aneuploids (non-mosaic) using PGT-A by NGS (n = 93 whole chromosome aneuploids, n = 7 segmental aneuploids). In addition to the original clinical TE biopsy used for PGT-A, each blastocyst was subjected to an ICM biopsy as well as a second TE biopsy. All biopsies were processed for chromosomal analysis by NGS, and karyotypes were compared to the original TE biopsy. PARTICIPANTS/MATERIALS, SETTING, METHODS: The setting for this study was a single IVF center with an in-house PGT-A program and associated research laboratory. MAIN RESULTS AND THE ROLE OF CHANCE: When one or more whole chromosomes were aneuploid in the clinical TE biopsy, the corresponding ICM was aneuploid in 90 out of 93 blastocysts (96.8%). When the clinical TE biopsy contained only segmental (sub-chromosomal) aneuploidies, the ICM was aneuploid in three out of seven cases (42.9%). Blastocysts showing aneuploidy concordance between clinical TE biopsy and ICM were also aneuploid in a second TE biopsy in 86 out of 88 cases (97.7%). In blastocysts displaying clinical TE-ICM discordance, a second TE biopsy was aneuploid in only two out of six cases (33.3%). LIMITATIONS, REASONS FOR CAUTION: All embryos in this study had an initial classification of 'aneuploid' and not 'euploid' or 'mosaic'. Therefore, the findings of this study refer specifically to a TE biopsy predicting aneuploidy in the remaining blastocyst, and cannot be extrapolated to deduce the ability of a TE biopsy to predict euploidy in the blastocyst. No conclusions should be drawn from this study about the ability of a mosaic TE biopsy to predict the karyotype of the corresponding blastocyst. Caution should be exercised in generalizing the findings of the sample group of this study to the general IVF blastocyst population. The segmental aneuploidy group only contained seven samples. WIDER IMPLICATIONS OF THE FINDINGS: The high rate of intra-blastocyst concordance observed in this study concerning whole chromosome aneuploidy contributes experimental evidence to the validation of PGT-A at the blastocyst stage. Concomitantly, the results suggest potential clinical value in reassessing blastocysts deemed aneuploid by TE re-biopsy in select cases, particularly in instances of segmental aneuploidies. This could impact infertility treatment for patients who only have blastocysts classified as aneuploid by PGT-A available. STUDY FUNDING/COMPETING INTEREST(S): This study was supported by the Zouves Foundation for Reproductive Medicine and Zouves Fertility Center. The authors have no competing interest to disclose. TRIAL REGISTRATION NUMBER: Not applicable.