Embryo quality and grading

With the exception of artificial insemination, fertilisation occurs outside the female body in most assisted reproductive techniques and later embryo transfer is performed.

For selective embryo transfer, specialists rely on a series of criteria which help determining the quality of the embryos.

While embryos are being cultured, their quality is microscopically assessed. Embryo quality assessment implies analysing both embryo morphology and embryonic development.

Embryo morphology:

• Grade I:

Cells or blastomeres are of equal size and no fragmentation is seen. Their texture is uniform and they are light-coloured. There is no evidence of multinuclear blastomeres (more than one nucleus per cell).

• Grade II:

Minor fragmentation only (less than 15% of the embryo) and/or cells are mostly of equal size.

• Grade III:

The fragmentation rate is 10-40% of the embryo. Blastomeres are of unequal size and/or their surface is less smooth.

• Grade IV:

The fragmentation rate is higher than 40%. Cells are now dark-coloured and less smooth.

• Grade V:

Fragmentation is heavy and the embryo is dark-coloured.

Grade I and grade II embryos are considered as good quality embryos.

Conversely, grade III embryos are considered as moderate quality embryos, although there are still chances for pregnancy. Embryo quality is related to embryonic development and embryo–endometrial linkage capacity. However, grade III embryos are not necessarily associated with a genetic alteration. Grade IV embryos present a poorer prognosis.

Grade V embryos are not transferred to the maternal uterus. They are cultured while their progress is monitored. Nevertheless, practically overall majority of grade V embryos do not continue its development. Their poor development may indicate that they are non-viable.

Regarding their morphology, the following aspects are also taken into account: the zona pellucida thickness, the cytoplasmic vacuoles, the adhesion between blastomeres and the presence of a cytoplasmic filament.

If the zona pellucida is especially thick, embryo transfer can be performed previously to the assisted zona hatching (AZH) technique, a.k.a. assisted hatching (AH). AZH may facilitate the inner cell mass to break through the zona pellucida.

Embryonic development is another marker for embryo quality assessment.

This criterion can be summarised as follows: the greatest number of cells, the greater probability of implantation into the maternal uterus, always respecting the maximum of 4 cells two days after fertilisation and 8 cells for 3-day embryos.

Embryos with a cleavage faster than normal are considered as poor quality embryos, since they are more likely to develop a genetic alteration, usually aneuploidy.