Embryo Grading

Definition - What does Embryo Grading mean?

Embryo grading is a technique that allows embryologists to observe and rate the development of lab fertilized eggs (embryos) prior to implantation during an IVF procedure. The grading is based on subjective assessment of the embryo’s potential to develop. The process is used by embryologists and physicians in conjunction with information on the patient’s health, age, fertility and medical history.

This information helps the professionals determine which embryos to transfer, how many embryos to transfer, and when to complete the transfer. It may also be used to determine which embryos to freeze during cryopreservation for future fertility treatments. Thus, embryo grading is an important factor in increasing the chance of IVF success.

FertilitySmarts explains Embryo Grading

IVF begins with the collection of gametes: egg and sperm samples from biological parents or donors. Once collected, the samples are joined together in a lab; and when fertilization occurs, the egg is considered an embryo. The embryo remains in the lab for a period of days before it is transferred to the woman’s uterus. This time delay serves to simulate the embryo’s natural experience of traveling through the fallopian tube before reaching the uterus, and allows for the appropriate amount of cell development to occur.

Typically, several eggs are fertilized during one round of IVF. This results in the production of multiple embryos. During each embryo’s time in the lab, it undergoes embryo grading to determine its viability in the uterus and growth trajectory.

Embryos are graded at two different points, usually either 3 or 5 days into development.

Day 3 embryos are called “cleavage stage” embryos, which refers to type of dividing (referred to scientifically as “cleaving”) happening. By day 3, the cells have divided, but have not grown. When analyzing cleavage stage embryos, scientists are looking for the following:

  • The number of cells
  • Consistency of cell size
  • Clarity of the cytoplasm inside the cells
  • The presence of fragmentation, or a broke piece of the cell

Based on these factors, the 3 day embryo receives a grade ranging from 1, an ideal circumstance with high consistency and no fragmentation, to 4, a highly abnormal cell. A score of 1-2.5 is the most likely to lead to IVF success.

Alternatively, day 5 grading examines a blastocyst stage embryo. The term blastocyst refers to the more advanced type of cell division being experienced. On day 5, scientists are examining the following:

  • Size in relation to zona pellucida (ZP). The ZP is the protein based shell that surrounds the embryo. In order to successfully implant in the uterus, the embryo has to break out of its ZP, which must have thinned to a penetrable layer in that time.
  • Presence of two types of cells, inner cell mass (ICM) and trophectoderm (T). All pregnancy development requires two types of cells, ICM and T cells. ICM cells are responsible for the fetal growth, while the TE cells are responsible for the other components of pregnancy growth, such as the placenta and other tissues.
  • Number of cells in the embryo and overall size.

Each clinic and lab has their own guidelines regading how embryo and blastocyst grading is done.


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