Steps of IVFFertilization:

The fertilization of the egg by the sperm after oocyte retrieval is commonly conducted by either two processes, conventional IVF or through Intracytoplasmic Sperm Injection (ICIS). Conventional IVF consists of mixing the sperm and the eggs together to fertilize while the ICIS process requires the injection of a single sperm into the egg for fertilization in the laboratory. This technique is reserved mostly for couples with male infertility or with a previously failed IVF attempt. Such examples of male infertility consist of inability to ejaculate, blockage of male reproductive system, low sperm count, poor sperm quality, or retrograde flow of sperm. ICIS can also benefit patients where there is a low oocyte yield, maternal age greater than 35 or have previously frozen eggs or sperm.

Embryo Development:

Embryo grading is used by the embryologist and REI physicians to determine the optimal day to transfer the embryo and which ones to transfer during the FET (fetal embryo transfer) process. The transfers of the embryo are usually done on days of development where the stages are clearly defined, which are on Day 3 (D3) or Day 5 (D5-D7)(majority of embryo transfer). In our lab, we culture embryos to D7. If any embryos could not reach the stage of blastocysts on D7, their development are compromised and delayed, these embryos will be discarded (standard practice).

D5 Embryo Grading: The 5th day of embryo development is known as the “blastocyst” which will have gone through multiple cell divisions to consist of 64-128 cells as well as formation of two distinct layers surrounded by an outer shell known as the zona pellucida. These two distinct cell layers consist of the inner cell mass known as the embryoblast which will eventually form the infant and the trophoblast that lines the outer layer which will form the placenta. The embryoblast and the trophoblast are graded via A, B, and C independently, with Grade A being rated as good and C graded as poor to form the final grade of the transferred embryo. When the embryos reach blastocyst stage, a couple of cells will be removed from placenta for PGT testing, if you choose the PGT testing. There are some key features that the embryologist and the physician will look for to determine the best embryo for transfer and these include the day5 or day6 of blastocysts cryopreserved, number of cells, amount of fragmentation, and the consistency within the cell. In addition, besides the quality of the embryo, the embryologist and the physician would also need to consider other demographic factors including the patient’s age and past fertility history. Overall, it is important to determine the grading of the embryo as it determines the success of the implant and eventual pregnancy.

The grading consists of a number and two letter grades (number, letter, letter). The number represent the volume and cells in the cavity.

  1. Early Blastocyst – Blastocyst Cavity is 1/3 of the volume
  2. Blastocyst cavity is ½ of the volume
  3. Blastocyst cavity is 70%-80% of the volume
  4. Expanding Blastocyst
  5. Expanded Blastocyst is partially hatching out of the zona (shell)
  6. Expanded Blastocyst emerged out of the zona (shell)

The first letter Grade which represents the inner cell mass (fetus)

  1. Many cells which are tightly packed
  2. Several cells that are loosely grouped
  3. Few Cells

Final letter grade represents the trophectoderm (placenta)

  1. Many cells with a cohesive inner lining
  2. Few cells with loosely formed inner lining
  3. Few large cells

Assistant Hatch: Assistant Hatching is an additional procedure performed during the IVF process. After the “blastocyst stage” of Day 5 of embryo development, the embryo will continue to grow and expand in size and eventually will stretch and thin out of the zona pellucida to enter a process of breaking free from the zona known as “hatching” to allow for the embryo to implant into the uterus. However, during the developmental stage, not all of the embryos are able to successfully “hatch” from the zona pellucida and therefore will require additional assistance. This process is known as “assistant hatching”, which includes having the embryologist make a tiny hole in the zona using laser.

Lauber, Sean. “Complete Guide to Embryo Grading and Success Rates.” Remembryo, 13 Oct. 2018,

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