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

To investigate the parameters of mosaicism and the biological mechanisms leading to healthy pregnancies from mosaic embryo transfers. Prospective study. IVF center and associated research laboratory. Fifty-nine patients. 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. Chemical start of pregnancy, implantation, fetal heartbeat, and birth. 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. 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. Cien embriones mosaico transferidos prospectivamente en una sola clínica: explorando cuándo y por qué dan lugar a embarazos sanos Investigar los parámetros del mosaicismo y los mecanismos biológicos que conducen a embarazos sanos a partir de la trasferencia de embriones mosaico. Estudio prospectivo. Centro de FIV y laboratorio de investigación asociado. Cincuenta y nueve pacientes. Los embriones se sometieron a pruebas genéticas en etapa de blastocisto preimplantatorio para detectar aneuploidías mediante secuenciación de última generación (NGS). Las biopsias de trofectodermo que contenían entre 20% y 80% de células anormales se consideraron mosaico y los blastocistos correspondientes se trans