Embryo assessment at the pre-compaction stage in the IVF laboratory

Introduction Historically, embryo transfer was performed during the cleavage stage, either on day 2 or day 3. With advancements in embryo culture, many in vitro fertilization (IVF) programmes have moved towards blastocyst transfer. The recent development of pre-implantation genetic screening (PGS), metabolic profiling of conditioned media and time-lapse microscopy provide additional information on embryo selection. However, the assessment of cleavage-stage embryos remains crucial in that the information provides embryologists with a preview of embryo implantation potential. This is especially critical for patients with a limited number of embryos, who do not have the luxury to carry on to blastocyst culture for transfer. At this point, embryo morphology remains the sole selection criterion for choosing embryo(s) for transfer. This chapter discusses embryo assessment with the intention of being a quick guideline for embryo selection/grading with respect to symmetry, multi-nucleation and degree of fragmentation. Background Embryo morphology is widely used by many embryologists as the sole selection criterion for choosing embryos for transfer. It provides a safe, non-invasive approach for embryo selection. Many features have been suggested in the past two decades to accurately assess the implantation potential of cleavage-stage embryos. Quality assessment of the zygote is discussed elsewhere in this book. In this chapter we will focus on the quality of cleavage-stage embryos, day 2 and day 3, roughly 48 and 72 hours after insemination, respectively. Several features, such as cell number, symmetry, multi-nucleation and fragmentation, are often considered to be important features when judging embryo quality. Cell Number/Cleavage Rate Embryo growth rate has been considered to be one of the most important characteristics when evaluating embryo quality. Sequential observations for normal embryo growth rate and developmental patterns during cleavage stages are commonly practiced by embryologists. For normal development, human embryos are expected to have four to six cells 48 hours after insemination and six to ten cells after 72 hours. There are several studies indicating that abnormal developmental rates have a negative association with pregnancy rates. Uneven Cleavage ‘Uneven cleavage’ is defined as the uneven distribution of cytoplasmic materials when cells divide, leading to the uneven size of blastomeres.