Embryo Grading
After your egg retrieval and subsequent fertilization, one of the most pressing questions you may have is “which embryo should we transfer? Which one will lead to a positive pregnancy?”
There are many factors that will lead to this decision. One of these factors will likely be the grade of your embryo. This is one tool used by fertility clinics order to choose the embryo for transfer which has the highest chance of becoming a baby. Fertility clinics grade embryos with different nomenclature but each grading system enables the team to distinguish between good, average, and poor quality embryos in order to choose the embryo for transfer which has the highest chance of becoming a baby.
There are two primary grading systems utilized by most clinics. These are based upon day of transfer after retrieval (Day 3 or Day 5). Why is this important?
An embryo begins its growth as a single cell, and then divides every 12-24 hours. By Day 3, it is referred to as a cleavage-stage embryo. During the early growth of the cleavage-stage embryo, all of the energy and chemicals required for cell division come from the mother’s egg. By Day 5, the embryo, now called a blastocyst, is about 70-120 cells. A blastocyst has differentiated and contains two different cell types. The first is called the inner cell mass, which develops into fetal tissue. The second is called the trophoblast or trophoectoderm, and these cells lead to part of the placenta. In order to form a blastocyst, an embryo activates it’s own genes through a process called genomic activation. Genomic activation allows the embryo to produce the energy required for further cell division, differentiation, and blastocyst formation. The ability of an embryo to use it’s own genes and make it’s own energy is not simple. Approximately one third of embryos are capable of successfully activating their genes and growing to the blastocyst stage. This means that only about 30-50% of day 1 embryos make it to blastocyst stage.
Day 3 vs. Day 5
Day 3 - Clinics that grade embryos on day 3 of growth typically use a grading system ranking the embryos on a scale of 1 to 5 with 1 being the best. Day 3 embryos ideally consist of 6-8 cells held within an outer “shell” called the zona pellucida. At this stage of development, embryos are graded on:
Rate of growth (number of cells)
Symmetry (similar size of cells
Degree of Fragmentation (amount of cellular debris)
Day 5 - Some embryos will grow into blastocysts. The cells in these embryos begin to differentiate into two kinds of cells: baby-making cells and placenta making cells that surround a fluid-filled cavity. Blastocyst grading is determined by:
Degree of expansion based on how expanded the cavity is. This is graded on a scale of 2-6 with 6 being the most expanded
Appearance of the inner cell mass (baby-making part) which is graded with either an A, B, or C with A being the best.
Appearance of the trophectoderm (which makes the placenta) also graded with A, B or C with A being the best.
Note: Blastocyst embryos that are graded after 5 days of growth are graded with a much more complicated system.
Grading Systems:
Grade 1 through 2.5 embryos seem to have the greatest potential for developing to the blastocyst stage. However, a grade 3 embryo may also be of good quality if its appearance can be explained by asynchronous cell division rather than by poor development.
Blastocyst Rating Broken Down:
NUMBER: The degree of the expansion of the embryo’s cavity, ranging from 2-6
2 = cavity fills 1/3 of the embryo
3 = partial expansion, fills 70% of the embryo
4 = fully expanded cavity
5 = embryo has expanded and split open the zona
6 = embryo has completely hatched from the zona
FIRST LETTER: The inner cell mass (ICM) quality.
A = well-defined clump of cells
B = less well defined; may be grainy in appearance
C = a few dark cells, appear degenerative
SECOND LETTER: The trophectoderm quality. The trophectoderm is the cell layer that makes the placenta and the membranes surrounding the baby.
A = many smooth cells, equal size, forming a neat layer
B = irregular cell layer, some grainy cells
C = very irregular cell layer, cells may be dark and/or grainy
Embryo Morphology
There is correlation between grade and successful pregnancy rate (untested embryos). Findings from a study listed here are as follows:
“Excellent” embryos had 84.2% chance of ongoing pregnancy
“Good” embryos had 61.8% chance of ongoing pregnancy
“Average” embryos had 55.8% chance of ongoing pregnancy
“Poor” embryos had 35.8% chance of ongoing pregnancy
Learn more about embryo grading and related literature here.
Reference:
Last updated - 2/18/24
