What is Embryo Grading in IVF? How embryologists Grade the Quality of an Embryo? Should we transfer poor quality embryos?

Embryo quality

Embryo quality describes the morphological characteristics of an embryo that are typical for the day of its development. It is a tool that allows embryologists to select those with high probability to implant. An embryo’s quality does not absolutely correlate with its health and embryos with abnormal karyotype could still be of excellent quality based on their morphology.

To describe the quality of an embryo we utilize a grading system that differentiates for Day 2- Day 3 embryos (cleavage stage) and for Day 5 – day 6 embryos (blastocysts). Different labs use slightly different grading systems, but they all use almost the same morphological criteria.

Day 2/3 embryos grading system.

On day 2/3 of their development, embryos are graded according to their cell number, the cells morphology, and the degree of fragmentation.

Day 2 embryos ideally consist of 2-4 cells and day 3 embryos of 6-8 cells.  Embryos that have cells of equal size and have no fragments are considered of top quality. Fragments are common in human embryos and originate from uneven cell divisions where a small amount of the content of the cell leaks out among the cells. Fragmented embryos can still implant and result in a healthy baby. However, as the degree of fragmentation increases the pregnancy rate is lower.

Blastocyst grading system

An embryo left in culture in the IVF lab until Day 5/6 should ideally develop to a blastocyst. Blastocysts are embryos consisting of 70-300 cells and look like a sphere (Figure 1). Blastocysts are consisting of two different types of cells, the inner cell mass that will develop into the embryo and the trophectoderm that will eventually become the placenta. As a blastocyst grows fluid accumulates between the inner cell mass and the trophectoderm forming a cavity called blastocoel.

Figure 1. Blastocyst 4ΑΑ (Expanded blastocyst of excellent quality)Figure 2. Blastocyst 5ΑΑ (Hatching blastocyst of excellent quality)

During blastocyst grading embryologists are looking at all the three different parts forming a blastocyst, that is the expansion grade of the blastocoelic cavity, the inner cell mass and trophectoderm quality.

Grade of expansion is usually described using a number from 1 to 6. A grade 1 blastocyst refers to an early blastocyst where the cavity has just started to form. Grade 3 blastocysts have a blastocoel filling >50% the volume of the embryo. When the cavity fills all the embryo and the zona pellucida (outer shell of the embryo) seems thinner we are referring to a grade 4 blastocyst. As the blastocyst continues to grow it starts to hatch from the zona pellucida and is graded as 5 (figure 2). A completely hatched blastocyst is called grade 6 blastocyst.

In order to describe the quality of the inner cell mass we use the letters A, B or C with A being the best.

The quality of the trophectoderm is also graded with A, B or C with A being the best.

In summary, a blastocyst grade is consisted off a number and two letters. The number describes its expansion. The first letter describes the appearance of the inner cell mass and the second one the appearance of the trophectoderm. (Figures 1 & 2).

Success rates when transferring average or bad quality embryos.

Sometimes couples do not have good quality embryos to transfer. Other times they may have used their best quality embryos in the past but there are still embryos of lower quality cryopreserved. Hence, they wonder if they still have a chance using those embryos. Many studies have shown that embryo quality is simply a predictor of success rate. Indeed, better quality embryos have higher probability to give a positive result compared to an embryo of lower quality. However, no one can predict which embryos will implant. Most of the embryologists have a story to tell you about embryos of average or even poor quality that gave rise to healthy babies. There are many factors that influence the outcome of an embryo transfer and embryo quality is just one of them.