Introduction

Despite the fact that the first IVF pregnancy ever reported was from a blastocyst (Edwards and Brody, 1995), the transfer of cleavage stage embryos has dominated IVF for decades. This was mainly due to difficulties in successfully culturing embryos to the blastocyst stage as the culture media used were not complex and did not completely support normal development. In the early 1990s, knowledge of the metabolic requirements of the developing embryo increased and both co-culture techniques and sequential media were introduced. This dramatically increased the proportion of embryos developing to the blastocyst stage and therefore the application of blastocyst transfer in clinical IVF. The main objective of blastocyst culture was to increase the success rate of IVF because of better embryo selection after genomic activation and/or better endometrial synchronicity. Blastocyst culture has also been used as a tool to select the most viable embryos in a cohort with a consequent reduction in the number of embryos transferred and the corresponding reduction in the incidence of multiple gestations.

As the popularity of blastocyst culture increased, so did the need for a morphological scoring system. The blastocyst grading system introduced by Gardner and Schoolcraft in 1999 was quickly adopted by the majority of IVF laboratories. Although the system does not cover all aspects of blastocyst morphology, especially aberrant morphology, it has been very useful in classifying the degree of blastocyst expansion as well as the morphological appearance of the inner cell mass (ICM) and the trophectoderm (TE) cells. The Istanbul consensus document (Alpha Scientists in Reproductive Medicine and ESHRE Special Interest Group of Embryology, 2011) is mainly based on the Gardner and Schoolcraft system with some exceptions. The Gardner and Schoolcraft scoring system was an early attempt to describe blastocyst quality. The degree of expansion (i.e. Grades 1–6) was thought to reflect both the number of cells present and the blastocyst's ability to form a cohesive barrier of cells (TE) through tight junctions which enables the blastocyst to utilize energy to regulate their osmotic environment. This morphological differentiation was thought to represent the developmental capability of the blastocyst. As the blastocyst expanded, a more detailed morphological ‘picture’ could be obtained allowing for distinction between the ICM and TE cells (i.e. Grades 3–6). According to the grading system, the ICM and the TE cells could be assigned three grades depending on the number of cells present and the cohesiveness of the cell populations.

The blastocyst grading in this Atlas is based on the Gardner and Schoolcraft system with only minor changes using numerical grades for ICM and TE instead of letters, enabling mathematical computations, e.g. mean values. However, their system did not include a number of important morphological parameters often observed in the IVF laboratory. Albeit of unknown importance, some of these parameters were briefly introduced in the Istanbul consensus document, i.e. the formation of cytoplasmic strings often linking together different cells and cell types. Cellular and/or non-cellular structures within the perivitelline space (PVS) and/or the blastocyst cavity have also been described. In addition to these parameters we have also included additional morphological features commonly seen when observing human blastocysts as described further in the following text and sub-headings.