Embryo assessment at the blastocyst stage in the IVF laboratory

Introduction Embryo quality assessment and grading are major procedures in in vitro fertilization (IVF) laboratories and determining factors for clinical outcome success. Transferring in vitro-cultured embryos at the blastocyst stage (generally day 5 to 6 after insemination), including elective single-embryo transfer (eSET), allows better embryo selection and has been associated with higher implantation rates and better clinical outcomes, especially in good-prognosis patients having high numbers (six or more) of top-quality embryos on day 3. Blastocyst transfer has become the preferred choice, replacing cleavage-stage (days 2 to 4) embryo transfer, among IVF laboratories worldwide. Recent advancements in embryo culture media, culture conditions and equipment, trophectoderm (TE) biopsy for pre-implantation genetic testing and the concept of selecting only a few high-implantation-potential embryos for cryopreservation have contributed to more clinics practicing extended embryo culture to the blastocyst stage. Therefore, blastocyst quality assessment has become an essential and important protocol. Developments in time-lapse imaging (TLI) in recent years provide a more dynamic assessment supplemental to traditional morphology grading. New, non-invasive embryo selection approaches using proteomics and metabolomics also hold promise. However, in most laboratories not using TLI and yet to apply emerging non-invasive embryo selection methods, blastocyst assessment using existing scoring systems remains crucial for decision making in embryo transfer, cryopreservation and discard. This chapter discusses essential steps for blastocyst assessment as a quick guideline and lists blastocyst grading criteria based on three commonly used scoring systems in the reference protocol. Background Embryo morphology evaluations at the cleavage stage (as discussed in Chapter 31) and the blastocyst stage are widely used by embryologists as the sole selecting criteria for choosing embryos for transfer and/or cryopreservation. Additional days of morula and blastocyst culture facilitate selection of high-implantation-potential embryos, which are more likely to result in viable offspring. A major issue in limiting the number of embryos transferred is the ability to accurately estimate the reproductive potential of individual embryos within a cohort of embryos. Important questions reproductive endocrinology and infertility (REI) clinicians and laboratory professionals today may have are what embryo selection technique will result in the greatest pregnancy rate, live birth rate and greatest cumulative pregnancy rate.