At Carolina Conceptions, we assess embryo development on Days 1, 3, 5, and 6 post-retrieval. On these days you will receive a call from one of our embryologists, updating you on the status of your embryos.
On the day of your egg retrieval (Day 0), we assess the maturity of all eggs collected. All mature eggs are either directly injected with one sperm through a procedure known as ICSI (intracytoplasmic sperm injection) or placed in a culture dish with a specific concentration of sperm for conventional insemination. All immature eggs are discarded at this time.
Day 1: Fertilization Check
About 16-18 hours post insemination or ICSI, an embryologist will examine the oocytes under a microscope and sort them into several categories. Oocytes containing two pronuclei have fertilized normally, as this indicates they have the proper number of sets of chromosomes. These embryos will be kept in culture. Abnormal fertilization occurs when oocytes contain only one pronuclei or contain three or more pronuclei; these oocytes are discarded. Any oocytes that did not fertilize, or that have degenerated, are also discarded at this time.
When the embryologist calls you with your Day 1 update, he or she will let you know how many of the eggs retrieved were mature, and how many of those normally fertilized. At this point we can only report whether normal fertilization has occurred – embryos are not given a “grade” at this stage.
Day 3: Assessment of Cleavage Stage Embryos
The next time the embryos are removed from the incubator is on Day 3 of development. Following fertilization, the formerly one cell embryos enter the cleavage stage, where cells divide without increasing the overall mass of the embryo. During this time period we are able to count the number of cells, or blastomeres, and assess the level of fragmentation in each embryo. Each embryo is then assigned a quality score of either good, fair, or poor, based on the number of blastomeres.
On Day 3 of development, we expect to see between 6 and 8 blastomeres; these embryos are graded as “good” quality embryos. Embryos containing 3-5 blastomeres and those containing greater than 8 blastomeres are graded as “fair” quality. These embryos are still developing but are considered either a little behind or a little advanced in development. Embryos that did not divide (referred to as “no changes”) or those with less than 3 cells are considered “poor” quality.
Days 5 and 6: Assessment of Blastocyst Stage Embryos
By Days 5 and 6, we would expect normally progressing embryos to reach the blastocyst stage. A blastocyst is an embryo with a fluid-filled cavity that has two different cell lines: the inner cellular mass (ICM), which forms the fetus, and the trophectoderm (TE), which forms the placenta. It is at this stage that we can give an embryo a “grade” consisting of a number and two letters, such as 4AB. We use the Gardner Grading Scale, as outlined in the table below. The number refers to the level of blastocyst expansion, the first letter refers to the quality of the ICM, and the second letter refers to the quality of the trophectoderm.
While embryo morphology is not a perfect predictor of pregnancy success, those embryos with a higher morphology score generally have a greater likelihood of producing a pregnancy and surviving a freeze and thaw process. Embryos that meet a certain criteria may be transferred, frozen, or biopsied and frozen on Day 5. Any embryos that are still progressing, but do not meet this criteria on Day 5 may be given one additional day in culture and reassessed on Day 6.
When selecting the best embryo for transfer, we first look at the grade of the ICM, as that is the cell line that forms the fetus and is thus the most important factor. If there are two or more embryos with the same quality ICM, the grade of the trophectoderm is considered next. For example, we would consider an embryo graded 4AA to be a higher quality embryo than one graded 4AB.
We do not expect that all normally-fertilized embryos will develop into good quality blastocysts. Embryos which do not reach the blastocyst stage, or that do not meet a quality threshold, are considered incompetent and are discarded.
| Expansion | Status | Description |
| Comp | Compacted Morula | Compacted cells with no cavitation |
| EB | Early Blastocyst | Blastocoel can be seen. ICM and trophectoderm can’t be distinguished. |
| 1 | Blastocyst | ICM and trophectoderm can be distinguished. Embryo size is similar with early blastocyst. |
| 2 | Blastocyst | Blastocoel fills 30% to 50% of the embryo’s volume without thinning the zona |
| 3 | Expanding Blastocyst | Blastocoel fills great than 50% of the embryo’s volume without thinning zona |
| 4 | Fully expanded Blastocyst | Blastocoel fills great than 50% of the embryo’s volume with thinning zona |
| 5 | Hatching Blastocyst | More than 10% of the trophectoderm is herniating from the zona |
| 6 | Zona Free Blastocyst | The blastocyst has escaped from the zona |
| Inner Cell Mass | Description |
| A | Many Cells, organized and tightly packed |
| B | Moderate cells, disorganized and/or loosely packed |
| C | Few cells, disorganized and/or loosely packed |
| D | No visible cells |
| Trophectoderm | Description |
| A | Many Cells; no necrosis or vacuoles |
| B | Moderate cells; very little necrosis or vacuoles |
| C | Few cells; very little necrosis or vacuoles |
| D | Very few cells; necrosis and/or heavy vacuolization |